Probably a problem with that specific card. I have an HD 3870 in a system running 4 wows and haven't had any issues. I'm getting great performance with medium settings. Both the HD 3870 and 8800 GTS have about the same peak power. Now, the previous HD 2900XT peak power was bout 50W more.

AMD is coming out with a dual GPU version of the 3870 and should be a great performer for the price.