I had the same problem. And as a matter of fact it's not uncommon for the 8XX series with stock cooling to run at 90C or a little more. Mine hit 105C before I cleaned it.
I replaced the stock cooler with a zalman and get 65C under load now.
I dont know much of ati cards, but 100*C is way to much
Also, make sure you're using ATI Catalyst Control Center. If you are make sure it's up to date. 😀 😀 😀 😀 😀