I had a mouse that would only move horizontally. Suspecting the 8520, I
swapped the chips. Then it would only move vertically. Turned out both
8520s were busted. I also had a bad Fat Agnus; I don't know if that
caused/was caused by/contributed to the problem. 8520s are less than 50
cents apiece and can be replaced by the novice. The Fat Lady is another
story entirely. — Mike —