Vintage Synth Repair

I have a Korg Mono/Poly MP-4.  I was able to tune each Osc on Circuit  
board KLM-398.  However, as you ascend the keyboard up from the  
lowest note to highest note, the pitch drifts up about a half a step  
or so on all 4 Oscs.  This is even on all and the same.  So, playing  
octaves sounds a bit off.

Also, noticed after repeated playing of octaves that perhaps one out  
of four repetitions will yield a pitch that is way off tune on just  
one of the osc.  There is no pattern here, it is random as to which  
osc is out of tune and when.

Lastly, some times if you play two notes together in POLY mode, and  
then quickly play a single note or let off the first note, you hear  
the note you just let off sound when you play the next single note.   
Like a double voice trigger.  Changing release times and sustain  
doesn't stop this behavior.

Any suggestions?  I'm not a tech, but a player who is willing to get  
under the hood and fix these problems.

TIA.