Emu XL-7 & MP-7 User's Group

I searched the archive but I couldn't find anything on this (or I 
didn't look correctly). I have noticed a couple of times now that my 
XL7, while just playing a pattern, and not doing anything at all out 
of the ordinary, will just lock up and give me this hideous buzz, 
although I should say the drums still seem to run, so maybe its just 
the synth sounds. Switching patterns doesn't help, I have to power 
down to get it to stop.
Any help GREATLY appreciated on this one.
Dana