geetee

You could also try a reinstalling your current Catalyst drivers and/or updating to the latest ones. It's possible the card and/or certain features aren't installed properly.

Secondly, try looking at the rear PCB side of the card to see if there are any LED's lit up on it. This would signal a problem with the card. Although this thread from the XS forums is a bit old, it's relevant. You might have a dud card in need of replacement.

To be honest, I have no experience with HD3870X2 cards. But they aren't too different from current designs.

Has the fan always run at 100% or is this something recent? If you're using CCC (Catalyst Control Center) you could also try it's manual fan speed control. If not, you might have to take a look at the two BIOSes on the card, to see what's going on.