Backfire

["JedPiper" <JedPiper@earthlink.net>]

Wally - 

A few hundred RPM isn't significant enough to indicate a problem with the
advance however, I would still swap distributors with your donor as a
diagnostic step.  
If you do try advancing the timimg as Mike suggests (old distributor
installed), and you find that the engine smooths out at around 23 degrees,
I would still look to the advance unit as the source of the problem.   It
may not be completely malfunctioning, but it's probably on it's way out.

Jed