anyone have Hard-lock-ups under stress over about 438 fsb ?

this is with a Q6600 ...

I need 1.5V for 3.4 gig ..

but anything above 438 fsb - and I don't get a PRIME error as such, but anywhere between 8 minutes and 30 minutes I get a complete hard lock-up, if you then press reset the PC turns off, and then won't post ... if after lock-up you turn off at bakc, and back on again it posts. (without overclock fail)

I have mobo volts as high as I dare, I'm on

Vfsb 1.40 (have tried 1.45V)
Vmch 1.45 (have tried 1.55V)
Vsouth=1.6V

running Ballistix 8500 at 1:1, currently 5-5-5-15, 2.15V

running 4x 1gb modules

mobo temps are quite high (is system temp southbridge ??)

even though case is well cooled ... system temp idles about 48C and goes up to about 55C under load worst case

cheers, Mark.