Vintage Synth Repair

I have an MKI here that's been driving me mental for the last little while. Basically any note you hit on the keyboard leads to a sweep at the highest possible audible pitch. You can actually play the synth normally, but the moment you let go of the key, it picks up from that note and sweeps to a higher and higher frequency until it's inaudible. I can slow the sweep down with the portamento slider.

I've done thorough tests in the AR / ADSR section and they all check out. I thought that perhaps the caps were dead and were not discharging properly on release but they did. I even removed the keyboard assembly thinking it might be a dead cap somewhere there. I applied just cv to trigger the synth and it did the same.  I then started looking to the LFO but you can control it with all the sliders normally. It behaves as expected while this sweep occurs.

I think I'm going to owe someone a case of beer here. :)

TIA