D4 magneto 4S6471 Wico XH-2836S - original or field replacement?

The 955 12a use the magneto AMERICAN BOSCH and WICO 4S6471

I installed new parts in the magneto today, fitted it to the tractor and tested its operation, which was fine. I replaced the points, condensor, distributor rotor, and distributor cap, with new parts from Standard Magneto. The fellow there said I should also replace the coil but I decided to continue using the current coil in case it was still good, which it was. So I have a spare coil if anyone needs on, at cost ($44).
The parts list is:
Part Cat# Wico# Price
Points 7M3894 1-5111 14.66
Condensor 2M2757 2-5007 9.63
Dist. Cap 5S5109 3-5025 55.72
Dist. Rotor 5M8036 4-5008 48.46
Coil n/a 5-5011 44.18

Things noticed - I needed to ream the bore of the points breaker arm as it was tight on the brass post, and I need to whittle the distributor cap as it was a bit too tight on the lugs that fit inside the magneto case for my liking.
The bearings in the magneto were tight and smooth so no other r-n-r was required other than cleaning and a new main gasket on the pony mounting surface

Looks like the magneto has become unreliable - will spark initially then after a few seconds of running (or maybe a minute) it will stop dead. After a minute or two I can get it to run again briefly. Condensor was new last summer. Does anyone have this mag lying around on the shelf? We need a new distributor rotor pedestal as ours is broken and glued back up badly