OBERHEIM SYNTH

Guys, I am still here :(
After rebuilding the PSU I had some day of "pause", then yesterday I had the time to play my matrix: after one hour the issue was back again... doh.
Today I made some additional test:
- voltages are ok (eh...)
- the issue is better described as a freeze of the synth, not a voice stuck: the keys you are pressing when it freezes stay on, but everything is no more responsive
- If I remove the ribbon connector on the voice board (before turning the machine on) after some variable time the issue appears, so I am prone to locate the problem on the cpu board
It's definitely a faulty component because the synth works starting cold.
Any suggestion on how to track the issue would be appreciated. I am suffering soo much :(