I have a 88 420 Gas engine. I just had major tune up work done to pass inspection. While driving on the interstate at 60 -65 it seems to "surge" . It is a very uncomfortable feeling. It is not as if the engine is going to quit. The car is thrououghly warm, and just when going up a slight incline. When I step on the gas for more power, the response is perfectly normal, but when cruising with a light throttle pressure and a little load it surges. My thought is perhaps the mixture is a little lean, or I hope it is not a ignition issue. Any thoughts ?