Dmitri, thank you so much for your help!
The problem was definately coming from the LEFT pushbutton. It worked only half of the time but I didn't figure that out immediately. I have changed the pushbutton and it now WORKS!!
I was even able to upgrade the firmware!! And most suprisingly, I was able to upgrade the firmware with my Mac (PowerPc G4)!
The only weird thing is that after powering while holding the LEFT key, I wasn't asked for the quartz I'm using (16Mhz, 20Mhz etc...). In the Config tool, the upgrade screen explains that we have to hold the left key THEN to press UP one, two or three times depending on which quartz we are using, but pushing the UP key did not show anything.
In other words, I was able to upgrade, but it seemed like there was no need to choose which clock I am using. I pressed DOWN and it worked! But has Megadrum chosen a radom clock.. I have no idea.
And by the way, now that I have upgraded the firmware I have fully access the all the menus!!!
FINALLY !!!