Hey Peter, thanks for your reply.
Good point about that Demux area. I have done a lot of work in that specific area a good while ago with the help of several cool people on this group and managed it, new chips, new traces and so on.
It's definitely worthwhile to re-check that again. I just know it took me a very long time when I did it, but I should be able to do a check faster this time.
The reason I thought of a potential ground issue is that there is in the default configuration, a GND lead which is soldered onto KLM-367. However, this makes it hard to work on the synth in stages as you need to solder and de-solder this each time, so I added a jumper which makes the connection easy to do and remove when opening the synth.
However, a jumper isn't very robust, so that's why I am thinking it could be intermittent here, possibly being pulled apart a little, breaking connection sometimes, sometimes making it when the faceplate is closed.
I'll try and make it a more robust joint and see if that helps. If it doesn't, it's off to checking KLM-367 again :D