Placing your hand a safe distance away from the tailpipe, are the exhaust pulses that come out smooth or can you detect it misfiring? If it's misfiring then you should check to see that your TPS is working correctly and your timing is properly set at 10 degrees BTDC. Beyond that you could have a distributor cap, spark plug, spark plug wire, etc. that is in need of replacement. Have you checked for trouble codes?