I had a similar problem, but more chronic backfiring through carb. It turned out to be a vacuum leak. Has this been a long term problem, or just recently after changing something like intake manifold or carb. I had changed both, and in the process did not screw down far enough the vacuum "bung" for auto-trans. The rear of the carb was sitting on the "bung". It drove me nuts I took off carb and re-installed it along with intake manifold several times. Based on my experiance I would guess a vacuum leak of some sort.