I've been having a big issue overclocking over the past few days
when I noticed that my mobo would not keep my O/C settings that I
had set up a while ago. (Back then, everything was fine.)

This is what I have :
Gigabyte P35-DS3R
4GB RAM (4x1gb OCZ SLI-READY 1033MHz)
C2D 6750

I have the F11 BIOS and after a few hours of trying different voltage
settings and different clocks, I would always come to the same
results :

Black screen during POST and PC reboots itself while resetting the
BIOS to default settings.

So I went one by one, trying to see which settings would be causing that.
I am aware that my ram runs at 2.1 so the voltage was set
since the start accordingly (+0,3-4) and was trying to keep
the ram as close to 1033 as possible.

I've read on some forums that it could be caused by the current setting:

PCI Express Frequency (Mhz)_______ [AUTO]. which could scale while
trying to O/C.

I then changed it to 100 as suggested everywhere but it seems to
be the source of the problem, as even if I put the BIOS to all default
(plus ram voltage) everything loads fine. If I change the PCI Express
setting from Auto to 100 or any other "manual" frequency, I cannot
ever go past the POST.

Anyone have an idea what could cause this ? Could it be my video card ?

Thanks in advance.