[CobraEd]

If you have the vacume advance hooked up, you can get a breakup at part throttle or when you are off the gas during these high vacume conditions. If you have your overall timing set pretty high, that extra advance that the vacume advance produces can cause the engine to break up and miss. Once you step on the gas, the vacume drops, and the extra advance from the vacume advance goes away. To test this out, just pull the vacume hose off at the distributer and plug it. Then drive the car and see if the problem goes away.

Ed