strange 7950gx2 problem

[SlartiBardfarst]

i have a very strange problem where i can not overclock my system (cpu\fsb) with my gx2 in place. i mean not even 1 mhz!!
i've had my system stable at more than 3.6ghz for weeks with my old 7800gt but once the gx2 goes in, it's stock or nothing.
the strange part is that when i put my 7800gt back in it's fine. i can overclock all i like again.

i know the card is fine as a) it works and b) it will sit in my mates overclocked pc (more or less the same spec as mine) no trouble.
i obviously haven't damaged anything during overclocking as it's fine with a 7800gt in.

i did think it could be to do with the motherboard not wanting to run the pci-e in 8x-2x mode rather than 16x-1x but others here have got the same setup and don't have this issue.

i'm out of ideas, you guys got any?

main system spec:
e6600 conroe
gigabyte ds3
2gig geil pc6400 ram
seasonic 430w psu

 

[w3bbo]

Your PSU seems a little underpowered to be overclocking a high end system.

 

[SlartiBardfarst]

it had crossed my mind.

although my thinking was that as it runs stock no problems with good rail figures, the fact that i ask 1mhz above stock would make no difference to the power draw.

it's always the same point at which it crashes. it detects the cpu frequency at POST, does a memory test and resets.
mobo is running the latest F5 bios although it also did this with the previous F4 one.

 

[SlartiBardfarst]

ok, this is getting silly now.

i've replaced my PSU with one that has a little more behind it, a corsair 620w'er so that can not be the problem.

it still happens though. change any clocking settings and it will not get past POST!!!!

HELP!!