Is it only on driver side? You could have a dead cylinder, ignition related. The fuel/air mixture igniting in the header/manifold. Get an infra-red temp gun(very useful), and check the temp at every exhaust port after running the engine for a while. If you have a dead cylinder, it be cooler than the rest. Then check the plug and wire on that cylinder, or cap and rotor.
PAWN SHOP FIND
RESTORATION IN 1980...
"You can have it all, my emipre of dirt. I will let you down, I will make you hurt"