A plea for help! Having spent hours trying to fix this over the weekend, this is driving me mad. If you have a working HD4870X2 on the DFI LT X38 please give this a read...

My HD4870X2 installs with no problems, and works fine in 2D. Start ANY game or a benchmark (i am running win XP SP 2) and the system crashes or gives perfomance which is unusably poor. Here the list of things I have done to try and sort this out - none of these have worked.
1. X38 bios updated to 08/21 (I have also tried 4 or 5 other versions).
2. System not overclocked in any way, all settings at defaults.
3. Tried all catalyst versions from 8.5 onwards, currently on the 8.8 beta, (i have tried the official)
4. Card has been RMA'd and replaced with a new one.
5. Fresh windows XP install
6. Updated all chipset drivers.
7. Change PCI-express compliancy option in bios to 1.0 (or any other pci-express looking option)
8. Different PSU (thermaltake 750w)
9. Different memory sticks
10. Move card to second PCI express slot
11. Use second physical DVI output on the card
12. Deactivated all other integrated devices on the board
13. Used driver cleaner to remove old drivers
14. Updated/reinstalled directx 9.0

I am really stumped here. Any suggestions most welcome.