If all caps were replaced for the capstan motor, did you check the pitch control switch, wiring, and also the variable + - adjuster pot?
Sometimes a bad selector switch and/or speed control can cause the drive system to behave very oddly, or not function properly.
I had one of these in ages ago and obtained the circuit diagram (so I'll see if I can find it for you) but I have to say many people retrograde back to the MkII (with a simple belt drive!) as the MkIII can be such a pain when it goes wrong!
(Thankfully) I never had to get into the awful 'Hysteresis Tension Servo Control System' on these.
Ah, the possible nightmares of modern tech......
