FSB 460ish might be getting up there for a Q6600. I know my Chip doesn't like it much, and I would probably have to do some serious tweaking to get it stable.

So I assume you are doing 425x9 for 3.8Ghz?

My BIOS shows two GTLREF values. GTLREF1 which defaults to .67 and GTLREF2 which defaults to .63 I believe. I can only assume that 1 corresponds to the CPU GTL and 2 corresponds to the NB GTL.

As far as I've been able to understand, the value for the CPU GTL ofg a Q6600 should be 67%. Perhaps try upping that one.

When I get up into the higher FSB's it helps to tweak these values and find a good combination. Anything past 450FSB and it helps me to up the GTLREF1(CPU GTL) +3%.
I'm not saying this will work for you, but it couldn't hurt to play around with these a little bit.

Are you sure it's not your RAM holding you back??? What are the frequency and timing values when you try your 3.8GHz overclock?