I had this problem with a 383 with 11.5 to 1's and fogger NOS set up plumbed into each runner and we installed in a 1971 demon 340 car, (factory 340 car) with hooker fenderwell headers and Direct Connection hydraulic bumpstick and a low profile offy intake 2x 4 set up with holley 650DPs mounted sideways on adapter plates that all fit under the stock dual scooped factory hood...
At first it ran like a raped ape and then it started to "happen" after we had about 90 runs on it..
We took it on the street and it started to find out what it was as it get's worse trying to find the issue...
It drove me nuts having not had it happen before and it turned out to be the valve springs!
I got one of those valve spring compressor's that allowed me to do it while motor was still in the car and one by one I changed out each valve spring with new ones that moparts sold as compatable and I used a small air compressor to keep the valve up in the head ( still I spun each piston to the top of the cylinder just in case I lost the valve so that it could not drop out of reach).
It never did it again after that.
Before this I had checked everything from my msd all the way to my tach as possible culprit's... Only to turn out it was the valve springs going away..
Grant Eaton 386-561-1181