If the timing light is lighting intermittantly then you have an ignition problem, possibly high resistance igntion wires, worn distributor, or slop in the timing chain causing spark scatter. I would ohm out the plug wires and check for slop at the distibutor.
I checked the vacuum line to the wipers...pulled it off the manifold inlet connection and taped the manifold inlet. No change.
Hmm...timing....I can see that the timing advances when the throttle is opened, so I assume that means vacuum advance is working. I did NOT disconnect the vacuum line when I set the base (idle speed) timing. Should I have done that? I'm not sure how that makes a difference, because at idle speed, there would be no port vacuum and therefore, no vacuum advance.
Hmmm. Most recently, problem has been in the rear cylinders...the ones with the longest wires..... But I have had intermittent firing and dead cylinders in the front, too, just not in this latest go around.
Well, I got it sorted out, with some professional help. The worst part of it was I had the plug wires for # 6 and 7 reversed...doh! That probably happened during one of my troubleshooting soirees. Installed slightly hotter plugs. The real culprit appeared to be the excessively old gas in the tank. Problem solved.