After a few months' hiatus I have managed to go back to my Falcon again. Basically I had the Falcon untouched, only transferred it from one place to another in a pretty safe bag.
To my surprise, even though it looked like working, it wasn't -- usually in the middle of a demo or clicking in COPS (XCONTROL clone) I'd get crashes, bus errors etc. The typical example was the WAIT demo, I could let it crash or reboot at the same moment reproducibly.
The interesting thing is that this behaviour could be observed even without any drivers and accessories. Just plain NVDI, which always worked in 060. So basically the SuperVidel made the Falcon crashing in 060 mode, assuming in a sort of old Videl register access.
I have verified this theory by removing the SuperVidel and yes, it helped.
Anyone has experienced something similar? I tried to mess with the J7 jumper but couldn't see any difference. Can't imagine what could have changed during the past 2-3 months since I have used it.