[Doug Tatham]

Is the engine vibrating badly? If so, check to be sure that the timing marks are correct and check the timing. If it runs smoothly, check your exhaust system and your catalytic converter. If it seems to bog down when you run at high RPM's it's probably the cat. If your exhaust is leaking there is frequently black carbon deposits on the pipes.
Very Important: Take it to a parts store that will check your charging system for free and see if the alternator is putting out the current it should.