Engine miss, drivin' me nuts

[walker]

I have an engine miss that is really bothering me. It is somewhat intermittent and so has been difficult to find. When the engine is cold it seems to run fine, most of the time, then starts to develop an erratic idle and stumbles up to 2500 rpm. After that it seems to smooth out. I had some trouble in the past with the mechanical advance sticking with the weights out, but I check that and the timing seems to be good. When I had it apart I looked at the cap and rotor and did not see any tracking, or burning. It is an HEI dizzy, on a 350, with a 1405 on a performer.
What is the best method of tracking down a miss? The engine only has a few thousand miles on it, and sometimes it purrs along just fine, so I do not suspect a valve problem.
Any ideas on this would be appreciated.