When you let off gas? Are you geared down driving or idling in the driveway? When I was moving to AZ I was going down a large steep grade just outside of Flagstaff on I17, it was a good 5 miles of grade, I was in 4th doing 65 with a 6x12 uhaul trailer and the motor was popping out the exhaust just because of the extreme lean mixture at the RPM's that I was rolling, it was such an extreme popping the junky flex pipe that was clamped onto my header flanges EXPLODED and I was running open headers the whole way to Phoenix without knowing it, had a bad headache w/vomiting that night due to exhaust poisoning....best gas mileage I got the entire 1500mi trip though....point is, is it a lean fuel mixture popping or misfire popping? Really sounds like you got weak springs in the distributor or maybe something in the carb...At idle, can you STOMP it will it rev up smoothly and come back down smoothly?
