I desuicided this original PCB but after booting up I got this
After checking rams and buffer they were all good.
Just to avoid any doubts I dumped without convinction all the graphics eproms which normally are very reliable.
To my surprise when I lifted eprom PWE02 with the pcb in operation, the parts of the background corrupted turned to black revealing that at least the problem was connected to the circuit around the eprom.
Placing the eprom on my programmer I got this:
Turned out that the VCC pin of the eprom was making bad contact internally.
Burned a new eprom and the problem was fixed:
A picture of the bastard before going to the bin: