is it worse on hard acceleration? That is the speed where you need the most spark under a heavy load. Perhaps a wire end arcing internally, or a coil that is not putting out its full 40,000 volts. Try spraying water at the plug wires (at the spark plug end). If the engine misses on one cylinder there, inspect the wires closly for arcing. To test the coil, pull the coil wire while the engine is running- the spark should jump strongly about 1/4 inch past the end of the coil tower. (Eliminates the need for a pacemaker if you hold the wire wrong)