Vintage Synth Repair

>>For arguments sake, what if the tuning errors are NOT stable, IE,
changing with time or temp as you mentioned?<<

check the connector from the keyboard to the main pcb. the part that
is soldered to the pcb is under some mechanical stress & the solder
joints can fail.
if both oscs are out by the same amount, it could be the keyboard
scaling buffer. see if the filter tracks properly (make it
self-oscillate & use a guitar tuner). 
measure the keyboard volts & adjust them to 1V/oct if necessary.

if the oscillators are both out /by the same amount/, then don't start
adjusting their tuning or scaling to remove an error that's being
introduced by something else.  

d.