Tach jumping is getting intermittent power to coil since tach runs off coil signals. While car is running if you put headlights on are they dim? Put a volt meter on battery and make sure you have about 12.6v on battery. I think the alternator on that year had internal regulator, so if it is going bad you'll probably have to replace alt too. DO you have a spare alternator around? try switching out and see if that fixes it. That intermittent stuff reminds me of a voltage regulator going out. Have the alternator checked at a local auto parts store , like Autozone or O'rielly's, it's free. They check batteries also. Coils usually just burn out, but very rarely. Make sure engine is grounded to frame real well from battery post, NEG.