The only chips I have ever had go bad in an Amiga are the CIA chips, so I doubt that your agnus has failed (but I have been wring before).
More probably a bad contact, but of course a bad chip is also possible. I have a spare 500 that used to need a push every now and then on the Agnus to get rid of the green screen. There was a buch of bad sockets giving trouble, cracking easily. Check the socket pins, if there is visible oxidation.
