Any backfires out of the exhaust when this happened? Just thinking (out loud) that it might have put a pressure pulse back up into the crankcase vai the EVAC system. I'd be checking the carb to make sure some raw gas is not getting out into the exhaust during the dieseling and exploding.
When you let off of the throttle, does the car idle down and give you good engine braking immediately? Or does the throttle seem to be hanging open a bit? Just checking for reasons that may point to a mildlly hung throttle... springs, bound throttle cable, etc...And too far open on the carb secondary stop may contribute to the dieseling.