3870x2 and X48T-DQ6 reboot at windows load screen

MoBo: Gigabyte X48T-DQ6,
Power: Antec TruePower 1000W
Memory: one 2Gig stick OCZ PC3 14400
2 1Gig HDDs
1 lightscribe drive
OS: Windows Vista Home Premium 32 bit
VidCard: VisionTek X3870x2 1GB

When I first built the system it worked but occasionally rebooted.
After about a month I powered on and it rebooted after the windows loading screen goes away but before showing the desktop.
It just kept rebooting. I swapped out the video card for an x1900 - and it works. I can boot Vista.

I told VisionTek and they RMA'd me (what they say is) a new card, but I still get the continuous reboot.

I installed XP with the x3870 and that loads to the desktop.
If I run dxdiag it freezes on direct3d test, and eventually reboots.
If I run the 'Call of Juarez' demo that came with the card (in XP) it works, but when I exit I see a message 'VPU Recover has reset your graphics accellerator", and the desktop has been resized.

One other thing. When I do a fresh install both Vista and XP start in 640x480x4bit, which is not only ugly but makes it hard to use the fixed 800x600 Catalyst install window Could that be an indicator of a problem ?

I have reset the bios multiple times. Installed the card in each of (two) PCI slots. Installed Vista with no updates, and with SP1.

Any help would be greatly appreciated.
I fear it is the motherboard ? - but then why does the x1900 work ? It's not dx10
I thought maybe the power supply - but it works fine in XP

A fresh install of Vista boots up, but as soon as I install the Catalyst driver - it no longer boots.
If I restore prior to the Cat install - I can boot again.
I tried the Catalys 8.2 that came with the card, and also 8.6 and 8.11 from VisionTeks site - all with the same result.

Can anyone help me ?

Thanks for the reply evongugg. Unfortunately I had already thought of that, but there are no Omega drivers available for Vista yet.