I finally opened this one up and took a look inside.
The PSU voltages looked fine on the oscilloscope and I couldn't see anything obviously wrong there.
I still experienced some resets/crashes but they seem completely random.
At one point I just left it and went and did something else. When I came back half an hour later it still hadn't crashed.
So, I played it a bit, tried editing sounds, no problems.
In the end I chose the Stab Brass sound and played the lead from The Final Countdown (hehe) and ended it with a solid double octave bass tone... and then it crashed.
Are there some bad connectors in there that glitch when I play the keyboard?
Immediately after resetting itself it reset again, without me touching it.
I opened it up again, removed all the cable connectors and put them back and tried pressing the EPROM chips into their sockets a bit in case they weren't properly seated.
Also, I tried looking at the data on the RAM-chips to see if the waveforms look like crap, maybe causing data corruption or something.
My shitty PicoScope 5MHz USB scope is barely enough to see those waveforms but they looked OK I guess.
Though I must have managed to do something wrong when I was measuring as I noticed all the sounds in the internal memory bank were trashed :(
Also, the tuning between the voices was horrible, as was the keyboard tracking. Every setting in it was just random garbage it seems like. Blah.
After realizing that just changing master tune fixed the tuning problem I tried playing it again and didn't see any crash for a while.
It was getting late and it feels like my only option now was to start replacing all the digital chips just to see if any of them caused it.
Barely worth it, huh? I just put it back together and back on the stand.
If it starts crashing again I don't know what I'll do.