Try swapping the 8520 CIA at U7 and U8, are U36 (7438) or U37 (7437) hot?
The Problem could also be the 8364 (Paula). Since the problem doesn't happen immediately but over time I suspect it is heat related and either the TTLs at U36 and U37.
I don't think the Gary is likely I've repaired a lot of A500's and never needed to replace the Gary.
