Can't reach FSB400 on GA-G33M-DS2R with integrated graphics

Here are my specs-
- Enhance 460W PSU (80PLUS certified)
- Intel Q6600
- Gigabyte GA-G33M-DS2R motherboard
- 2 x 1GB PC6400 Kingston ValueRAM (dual channel mode)
- Cooler Master Hyper 212 CPU cooler

I can run the system stable at 3GHz (333 x 9) with the RAM at 833MHz with the motherboard's integrated graphics. But at FSB400 and FSB401 (~3.6GHz CPU speed), the system boots up (during POST) with corrupted graphics for a few seconds then reboots at 2.4GHz CPU speed (the result of Gigabyte's built-in feature against failed overclocking attempts as mentioned in my motherboard's manual).

At FSB400 (or 401), the system memory will run at 1000MHz. I managed to run my RAM this fast with +0.2V memory overvoltage at FSB333 (after adjusting the memory multipliers in BIOS), so the RAM could not be holding me back. I even tried upping the CPU (up to ~1.415V), FSB and Northbridge voltage a little bit but to no avail.

I have suspicions of the motherboard's integrated graphics. But I don't have a discreet graphics card to test it out.

The GA-G33M-DS2R is one of the best mATX boards for overclocking, but I can't even reach FSB400. Is my integrated graphics holding me back? Or am I unlucky to get a Q6600 that doesn't overclock so well? How have other overclockers fared when overclocking this motherboard with integrated graphics?