Discussion about the Korg PolySix synthesizer

here's the thing:
i'm not repairing the polysix myself but i'm still pretty involved
since the man repairing it doesn't have that much experience with
analogue synths. and i have zero experience with actual physical work
on synths :/

so - the polysix:
the battery leaked but the traces were repaired. this guy probably did
it all correctly but i'm not 100% sure. let's say they're alright now.
we also replaced IC31 (the lower of the 2 identical 4xAND gates) but
the symptoms are still the same as they were before the repair:
- synth usually works okay for the first half hour or so. except the
patch storage.
- patch programmer: bank 3 and prog 3 are lit all the time. you can
select another bank and program but nothing can be saved.
- the attenuator 13 way switch doesn't work at all.
- after some time other controls stop responding (and stay like they
were set): waveform selector, VCA gate mode and effect selector.
- usually when this happens some program and bank leds (actually bank
1 and 2 and program 2) start dimly blinking like there's some short
circuit or something. this gives me doubts whether it's really CPU
(IC21) fault. if it were faulty it would probably just die or
something... right?

so - what would you guys check next? how possible do you think it is
that it's the CPU? what about the 18-pin SRAM IC? any other ICs that
usually fail?

i really appreciate your help so far!

thanks!
jure