It looks like you're new here. If you want to get involved, click one of these buttons!
Apparently, there are some stock cooling issues with the 4850's. Some suggest removing the fan, reapplying thermal paste and reattaching the fan.
I stumbled across another temporary fix to this issue over at techreport.com. Here is the link to the post for those of you comfortable with making your own ATI catalyst profiles and such:
The downside of this fix according to the article is that you have to set the profile every time you boot your pc, but supposedly it resolves the issue for now.