I swapped my 680i for the 780i years ago. Most mobo manufactures allowed this for free when it came out they weren't going to support 45 nm quads. The 680i I had wouldn't OC the Q6600 like the 780i. I could still get back 3.0 though myself. Never 3.6... let alone 3.8. Again, I could never get stable past ~3.3 on a x9 mulitplier. Had to drop to x8 and kick the fsb up instead.
Vtt at 1.2 may not give enough juice when you go over 400 (1600 fsb). On some boards apparently 1.2 is sill good for the 400-450 range. I'm running 475 (1900) so 1.3 was needed. I also needed more volts on the South Bridge. I'm not sitting in front of my computer now, so I can't remember what term that is. It can be one of a couple things based on the manufacturer.
As my 680i crippled OCing, your may just not want to push past a certain point as well. the E6600 went up to 3.6 w/o a hiccup. The Q6600 topped out around 3.2 ghz. A couple last things to look at. Verify you are definately not OCing your ram while you're OCing. Find out you SB term and give it more volts. Mine required a *lot* (1.5v I believe... right up to the red numbers) to hold 1900 fsb. As you're going higher, even if you're not that high, more may be required. It too low, I'd either not post, Windows wouldn't load or, if I made it into Windows, doing anything that put load on the memory would crash it by freeze or BSOD. Good luck.