5870 Crossfire issues - compatibility problem?

[Oh my days]

Hi,

I recently purchased a second Sapphire 5870 and installed it in a crossfire configuration with my original Sapphire 5870. Since then I've had multiple issues including:

Hard freezing
Multiple BSODs (Machine_check_exception / a clock interrupt was not received on a secondary processor)
Performance dropping in games after an hour or 2 of extended use

Sometimes the errors/freezing will occur midgame, other times at the desktop soon after boot and also on logging out a user after extended use (wtf?)

I've tried the following:

Reinstalled latest catalyst drivers from scratch including CAP
Updated the motherboard BIOS
Loaded failsafe BIOS settings
Increased cooling capabilities of the case (avg GPU temp is below 75-80 under load)

Most tellingly, when I remove the new 5870 the problems disappear. Also, if I install just the new card by itself, the problems re-occur. To me this sounds like a straight up faulty card, but I've sent it into the place of purchase and they've advised me that it's not faulty after stress testing it for days.

I'm just wondering if it's perhaps a compatibility issue either with my mobo or second GPU (would having a different GPU BIOS level affect anything? would flashing it help?)

Machine specs are:
i7 920
6GB Corsair XMS3 1333 DDR
Gigabyte UD3R Revision 1.6
2x Sapphire 1GB HD 5870s
OCZ Vertex 2E 100GB SSD
Corsair HX 750
Windows 7 64bit

I haven't bothered reinstalled Windows as the machine was rock solid prior to installing the second card and can't really see it being a software issue. Any suggestions would be appreciated.

Cheers.