Sounds like a typical Polysix with traces (and possibly ICs) damaged on KLM-367. Fixing KLM-367 properly should resolve it completely.
The installation of the Kiwisix upgrade should not only make you have a working synth, but will also add several excellent features like higher patch storage count, full MIDI implementation with high-resolution external NRPN control with 4096 values, etc...
Yash