Alright this is Frustrating.

[*hotrods*]

What type of distibutor do you have?

My brothers car was doing the same thing and it ended up being the HEI ignition module. We even had the module tested at the local parts house and it checked fine. We ended up replaing everything in the distributor except the module and it still did it. I decided out of frustration to change the module and its been fine ever since. Also check the grown in the distributor, a buddy of mines 66 Vette acted this way and I found a bad ground in his distributor.

Replacing the module will most likely fix the problem. What happens is the module heats up and then quits working. Don't waiste your time checking it, I would almost bet my car that's what the problem is.