okies, sounds silly, but also pull your Kickstart ROM out of its socket and reseat it.... sometimes the kickstart can have intermittant contacts on one or more of its pins, pass all start up tests but get flakey results like random crashes, garbage on screen and display/sound corruption.
Although you also have sound issues, I would also suggest running a memory checker thru your chip ram, a faulty chip there can really play havock with the Amiga.
best of luck fault finding
