Korg DW8000, Mono/Poly, MS-20...

a crippled dw8000 was recently passed on to me. at first i thought it 
was the key contact problem i read about, but further inspection 
revealed that within the first minute or so of turning it on, the keys 
do trigger and also it will trigger thru midi. the problem is that the 
output sound is complete distortion and not related at all to the actual 
programmed sound. i know this because every once in awhile i will hear 
the real sound for a second or two. the distortion will carry on for a 
couple minutes, then fade into silence. somtimes the distortion will 
return thru continued playing of the keys or midi input, but it always 
cuts out or fades again. if i switch it off for a few seconds and back 
on, sometimes the distortion will appear again. the battery seems fine 
as parameter changes are retained in memory. can anyone suggest what 
these symptoms indicate and what i need to do to fix it? could it be a 
faulty power supply?
thanks,
shayne