280C Running on only 5 Cylinders....HELP!

[6cndmbz]

Hi All,
Alright, well it wasn't the smartest purchase anyways, but I bought a 280c for $400. It ran well enough to get me back from San Francisco to San Luis Obispo (250mi) but has a few problems to say the least. I finally got it to stop smoking (vacuum diaphragm) but it still has a significant rough idle. If I rev it, it goes away, but if I put it in gear, it shakes the car violently and usually dies. I can drive it with two feet keeping the revs up, but if I stop, it dies. I put in new plugs...same problem. I pulled the wires off the distributor one at a time and figured out it is the number 6 cylinder. I borrowed a wire and swapped it in, same problem. Its not the plug and not the wire and its still not working. Does anyone have any idea what could be going on??
Thanks
Scott