iMac 27' i5 late 2009 don't boot.

My iMac 27' late 2009 don't boot, the screen become grey with only the Apple's logo.

He can boot on the USB Lion installer but not on installed system.

When I want to re-install the system, the screen become black after some minutes.

I've tried to reset the SMS, the PRAM/NVRAM, to change the HDD, to make fsck, to repair the disk with disk warrior and disk utility, to change the RAM memory...

HELP !

Max.

PS : I've made an AHT but all is OK.