I had a one time issue with the bolts coming loose on the rocker arm supports. Driving home the engine started to stumble and cough through the carb. Later inspection found an exhaust valve that was not opening(the rocker arm never touched it) the pressure on every other rotation was almost enough to stop a truck still in motion. When the intake opened, not a lot but some, it would vent through the intake. Then the next time around, a new charge would fire and be trapped. Was harder to find than you might think. If it had gone on when I was driving faster, well I think I might have seen the same thing.

Steve H