Good morning John, just a thought and my apologises for posting this late ,it's slightly similar to Goldy's udea but in view of all the work you've done and this problem just wondered if you'd checked out your oil tank for any sediment /debris .The only probem I have encountered on my engine was a stuck exhaust valve ,like you didn't damage the piston but required replacement . It happened after a very short distance approx 1 mile following the complete rebuild somehow I'd missed some debris in the tank despite I thought cleaning it completely . The whole rocker assembly needed to be cleaned out and after that haven't had any further probs. Seemed to me that the rocker feed was surceptible to collecting this muck and like goldy's thought restricting flow with serious results, hope it's fine ,best wishes BobH