Hey, once again, solved the issue, thanks for your advice ! Actually, after the flash, my AMD drivers wouldn't recognize my cards and therefore the fans won't work.
I installed a patch and everything is OK again.

I now overclocked and undervolted the cards with OverdriveNtool, everythings is OK, I manage to push my cards up to 30 - 31 MH/s, I want to chech if my cards do not have any memory issues, hence I install HwINFO64.

But surprise, surprise...

Another problem... 

My computer freezes when I launch the tool... 

Any idea ? I tried to uninstall / reinstall. Runned it in sensors only mode, normal mode... It crashes when it scans ATI scan #93 (??)