I have a 2F engine with a GM HEI distributor. I can time it in the shop and it idles great, and will run smoothly at all throttle settings for about 3 minutes. Then it start s stumbling and quits running. At that point the timing is found to be 4+ degrees off. Resetting the timing starts the process all over again. Distributor gear appears to be in good condition. Any idea what might be happening? How likely is it that the mating gear is stripped, and if so what's the best way the replace it? Valve timing does not appear to be affected by this issue.