I recently built a system as follows :

Operating system Vista 64bit

ASUS P5E3 Motherboard

Pentium Quad 2.2Mhz

2 Radeon 3870 in crossfire

4x1GB OCZ Memory DDR3 1066mhz

550Watt Power Supply (think Thermaltake)

I have been playing LOTR online and COD4 and have seen some crashes intermittently - not enough to warrant full investigation. However since playing Age Of Conan the system is regularly crashing - I have updated the drivers on ATI - the update having only just been released - however I think there is a problem with the graphics cards as I get slow down and also the display phases in and out as the AI on the mobo changes the graphics settings.

Anyone know of any issue with these cards in crossfire mode and/or with this motherboard?

You didn't list all the componets in your system but first thing that pop's out is the PSU. 550watts is a little low for two 3870 cards in one system.

Stoner is right. Sounds lik the PSU is not getting enough power to the cards. When those cards go 3d, they require a lot more power. I know most specs say min 450w, but that is a perfect situation. Fans, USB connections, opt drives, multiple HDs, etc, will all add to the load. You can easily eat up 150w.

I had the same problem with COD4 with one 8800gt and a 500w antec basiq. My psu was going out. Everything worked fine, but after a while things would slow way down got choppy, 10FPS. I tried other, more demanding games and it would crash. I got a new psu, all is good.

I would recomend you at least a 600W PSU, and maybe more depending on your cooling system. As a Quad core already drink lot of your power, and even more if you overclock.

When a iddle, the cards don't take so much power, and on low graphic games neither, But on your high graphics games, your 3870s can't get as much power as they need to run the performance you're asking them, so they fail. I suggest you to take a higher PSU that you need, so you'll be ready for your nexts upgrade, and for maybe future oveclocking.

Thanks for the input guys, have changed my PSU to a 700w supply and don't seem to have any problems since.