I had the same problem, I could not find the vacuum leak, I had a post or 2 on it.
I changed the spark control valve, and it took care of it.
(ok, so could the problem with the spark control valve have been the leak?)
excerpt of an old post....
"find a post on a diff forum, someones describes my issue to a T. He fixed it by changing the spark control valve. I had a rebuild kit handy, I changed my spark control valve. The car now starts, idles, warms up and stays running(whew)."
Good luck, I know how frustrating it can be. I wasn't confident enough to rebuild my own, I wasted many months mailing it back and forth to a rebuilder.
1966 Sprint 200
We got a full tank of gas, half a pack of cigarettes, it's dark... and we're wearing sunglasses