I just finished to open up the whole console, I checked every single part, I even opened the GDROM drive to check the black flat cable (it's firmly attached to both ends). Everything seems to be in order, clean, and stable (I found no loose screws).
The fan spins happily, the drive's moving parts are clean, no dust or dirt in the gears...
I put everything back in place but nothing changed.
Maybe I should try to swap the GDROM with another one pulled from a working DC, just to understand where the problem lies.
When playing Sonic Adventure, it happened that it hung just after defeating the first monster (Chaos 0). The background music kept playing, I waited some 10-20 seconds then as I was about to press the "open" button, the game went on, like nothing happened.
Whoa, this really confuses me!