Well well well, good news! I finally had all the components in my mailbox. Replaced all electrolitics and voltage regulators and, voilà, the machine is working as it should! I would bet those 78005 where the culript... Read somewhere thwy are prone to fault. Replace them with common 7805.
Now I will play my MX6 for some week to see if the solution is rock solid ;)
If you will not read me on this topic, good, everything is fine.
The final message: if the behaviour ofnyour matrix 6 is strange, refurbish your power supply first!