That much temperature differential between top and bottom tank indicates radiator passages a clogged plus bubbles indicate your getting compression in the cooling system.
Here is some reference info on how things should be....
 

A liner or sleeve may have a pin hole in it also. Air bubbles would indicate compression getting in the cooling system, cracked head, bad precumbustion chamber, bad head gasket.

Hi westoz48a,
seems like a part of your issue would be lack of coolant flow as the temperature differential between the top and bottom tanks is way greater than the maximum allowed in the Specs--see the Delta T scan below-- of 15 #F (8 #C). Normal expected is in the range of 8 to 10 #F

It is not unknown for the oil cooler core tubes of both the engine and the power shift transmission oil cooler tubes to clog with rust flakes etc--likely if the fan has been turning slow due to belt slippage and the engine boiled and stirred up sedimented rust/dirt etc. The radiator top tank cores can also clog due to this boil up too.

The coolant flows from the water pump outlet from the radiator bottom tank into the engine via the oil coolers firstly.

Suggest as a start to remove the coolers and check the tubes for blockage, repair the radiator relief valve assembly--the screw rust into the body and usually need drilling out very carefully. The rust comes from using the split spring washers which allow the water to enter the voids.

After that we will see if you have the likes of loose pre-chambers, rusted thru pre-chambers, cracked heads or simply a failed head gasket due to the overheating that has occurred if overheating continues after this work is done.

The charts below have data on expected air flows thru the radiator core with the engine idling at its specified low idle speed of :- 525 +/- 10 RPM.
Keep us informed so we can help further if needed.
Cheers,

as i suggested earlier clean the rad core. and use a radiator flush.
the rad may have to be dipped in a hot tank to clean it fully. any rad service should be able to do that.

test strip used to be available to test coolant for hydrocarbon contamination indicating a crack or failed gasket between cyl/combustion chamber and water gallery. test kits are available i do know that. repco has a kit for $180 im sure you could get one cheaper than that elsewhere. it would atleast help narrow it down a bit. i would do the test before flushing the rad you want to run this engine as little as possible.
it could be a air lock some where and its not bleeding properly. some engines are just a pain in the arse to bleed.

Hi Guys, thanks for everyones help to find my dozer overheating problem. I'm hoping its not a blown gasket or worse, so will look at tightening the belts and cleaning the radiator first and go from there.I will have to clean the radiator in house first as it has the standard West Australian tree protection canopy of the day ,stopping me from lifting the radiator up and out.
I am still puzzled why i could be running the dozer for a couple of years without any problems, then one day i get in it and it just overheats,i guess maybe a radiator or oil cooler blockage, or headgasket being a sudden overheating issue or should i say an over pressure rising issue.
I will look at getting a test kit to determine whether a headgasket,cracked head problem and saw the one online from repco as you mentioned trainzkid88, i'm not sure if there is one out there for my bigger cat radiator but will look into it.
The temperature difference in the radiator is a problem so i'll try and flush it out although the coolant looks clean as.The oil cooler might be worth a look at as you suggested edb and will look at that before pulling heads off.
I'll keep everyone updated to see if i can find a fix and maybe help someone else if they have the same problem as me with their dozer,Cheers.

how the kits work is you take a sample of the coolant adding it to the reagent and wait for the change if it changes colour you have a problem no change then no combustion leak.
they work on any liquid cooled engine.

it could be a sudden release of crud blocking the rad or oil cooler it does happen.

try back washing the cooling system ie put the hose in the top hose to rinse any loose crud out. generally they flow to to bottom so hose it the other way. proper flushing tools are available that are assisted by compressed air

I had another go at my dozer today,i managed to tighten the fan belts,even though it says they are self adjustable in the manual,one of the previous owners had modified it so they can be adjusted.
I then started to drain the coolant with the intention of cleaning the radiator out with a high pressure hose but changed that plan as the coolant is as clean as new so i am thinking maybe its not a clogged radiator,and coolant here is super expensive here,so i topped it back up again.
I blew the fins out with air,there was a fair bit of dust,so maybe that will help.
I also checked out the top overflow valve,the spring was fine but the overflow line was blocked and fixed that.
Well,before i would start it, i checked the oil,their was alot of moisture on the dipstick, but the oil looked fine,so i started it and run fine with no white smoke.
After it started to warm up,i did notice coolant dripping out from the radiator cap but would stop every now and again.I kept it going and pushed some dirt for about 45 minutes,the temperature gauge stayed at about a quarter,the water pressure gauge rose to about 3/4,but i am beginning to think it is a little inaccurate even though it is newish but has never worked perfect.
So it still seeps coolant out the radiator cap when warm.
I checked the oil again after running it for 45 minutes and noticed heaps of moisture and the oil a little bit milky,and near impossible to check the oil so i cut the motor straight away.
Now its probably not looking too good and fearing the worst,a head gasket or more. I am thinking to change the oil first and try again as i did powerwash it a few weeks ago and also it has been raining non stop for weeks so maybe water got in somehow.Also,would the green coolant become clear if it was leaking into the motor as what i can see looks like just water.
I did notice today a lot of moisture coming out of the breather,more than just blow by.
So anyones thoughts or answers would be greatly appreciated.I did write a long reply the other night but could'nt submit it on the forum as it said i had been too idle and had to logout and then the message disapeared,hope this one works,cheers.

When the motor will be very cold ,try to loosed the oil plug drain 1 or 2 tread you will see if some coolant is in pan and you could check if the fan pully are clean with no greace and oil in it could make the fan turning too slow.