Re: Help !!!!!
I had a problem exactly like this. Punchline - it was the pickup in the distributor (large body HEI).
What had happened in my case was that, somehow, the pickup in the distributor had come loose internally in a weird way. Once I got the RPMs up to a certain speed, if I would let off the gas, it would pop and sometimes stop firing completely. Hitting the gas would cause it to stop popping and fire back up.
The vacuum advance unit, when getting the full vacuum from letting off the gas and coasting, was pulling on the pickup and causing an internal problem with the magnetic pickup when it pulled it fully extended. Unplugging the vacuum advance unit made the problem go away.
Replacing the distributor completely cured the problem. Afterwards, during a post-mortem, you could visibly see the problem with the pickup in the distributor.
If you have another distributor to try, drop it in and see if that resolves it.
1972 Chevelle, 454, 200-4r non-lockup, 12 bolt 3.73
1981 Camaro Z28, 355, TH350, 3.73