Unexpected reboot if X server doesnt start soon enough

I use a 9800 gtx.
On startup if the X server doesnt start right away the computer will halt.
This become problematic when fsck feel the need to check disks.
The problem occurs with live CDs whitout nvdia proprietary drivers as well.

I think it might come from overheating. I’am looking for solutions to turn on the fan at 100% before X start.