@pbareges: I think I am having the same issue. Let me give you some background. I installed mine yesterday but left the case off while I was testing it. Everything worked great all day and this morning I posted a raving review in this thread about how happy I was.
So today I made a mount for the DSUB 9 adapter using an old VGA card backing plate, re-assembled my system and booted up. I noticed that my graphics were flickering all over the place so I took it apart and re seated the board. Same thing when I booted a second time. It looked a lot like the "Warmup Flicker" I was getting when running in multiscan productivity on my multscan monitor with the RGB/VGA adapter. So I waited about 20 minutes and came back into the office and sure enough the flicker was gone. So rebooted with the keyboard and bam, no video signal. Turn it off for a minute and sure enough the video signal is back. Now when I try and reboot with the keyboard to select PAL video mode (holding the mouse buttons down) it lets me do that but after selecting it and telling it to boot, I get no video signal. Turn the Amiga off for a minute and back on and there is signal...
So aside from the keyboard reboot, maybe try changing your display mode from PAL to NTSC or vice versa if you are in North America and see if it loses signal after reboot. Mine does. I don't remember it doing this yesterday so it very well could be that it is not seated correctly today. I have re-seated it about 20 times and nothing is making any difference.
Hopefully somebody else has some ideas.