Do what Save2600 ask. The problem is in the Agnus chip.
Probably just some oxidizing due aging (5 years in the attic).
The A500 is the most returned-in-guaranty computer C= made. Most resellers just made one thing (away from the costumers, of course): lift the A500 about 20cm from the table and drop it flat. I don't recommend this method of fixing the machine, BTW!
