This is an update to this post I made ~2.5 years ago. Anyways, thanks u/Einsteins_coffee_mug and u/Lurkmcgirk for commenting your solutions and suggestions. I took the unit to a local synth repair shop recently and the problem was the volume knob - they were able to clean and lubricate that knob and then reinstall it without replacing it and I can now get stereo output; 7 other jumpy and wobbly knobs were given the same treatment.

The repair also appeared to rectify a problem I had playing MIDI notes from a sequencer or a really fast arpeggio, so I guess the knob values vacillating generated so many events the synth got overloaded and stopped processing incoming MIDI data for non-consistent periods of time. Nowadays, I am having no issues using the synthesizer, so I will be able to make more use of it in the future. :)