Vintage Synth Repair

Hi all,

I just picked up a AJ2 for a good price. It had some mechanical
problem which I fixed. However, it has one problem that I am not sure
how to approach. Any advice/solutions gladly accepted...

The sound is good when 1 or 2 keys are simultaneously depressed.
However, when I depress 4, or sometimes 3, keys, there is an audible
scratching sound.

I have tested this on different setups, ie headphones, amps, still get
the same sound though. I've used contact cleaner on all the contacts I
can find, so I'm assuming this is a component issue.

Any thoughts?

Regards
Evan