[PeteF]

Could be leaking intake manifold gaskets. With engine warmed up and at idle, close the idle mixture screws one at a time. If the engine does not die, or stumble badly, you are leaking the air past the intake gaskets. Can also check this by squirting carb cleaner along the intake gaskets at idle and listen for a change in idle. And, verify intake manifold bolt torques are to spec, I think 18-20 ft-lb.