Intermittent engine miss

I have a 1991 lumina eurosport, 3.1 V6 engine, 222,000 miles. After recently replacing the intake manifold gasket, o2 sensor, PCV valve, and changing spark plugs I have started experiencing an intermittent engine miss at both idle, acceleration, and freeway cruising speeds. The car will run fine then suddenly I feel a significant loss of power and hear and feel engine missing. When I come to a stop the engine will shudder and tach needle can be seen shaking wildly at about 600 rpm. When I accelerate the transmission feels like it is bucking and the engine rpms rise slowly and hesitantly until I blip the throttle and engine seems to go back to normal running for 5-10 minutes then starts missing all again. Any ideas what could be causing this?