Ok well check the fan speed in Catalyst Control Center.
You may have to manually kick up the fan speed. I'm having the same issue with my hd5850 however I'm not overheating at all like you are. My temps only hit 75c while gaming with the fan speed defaulted to 21% and I have a pretty good overclock on it.
If I manually kick the fan speed up to 60% the temps don't go over 52-55c.
Im already make 100% speed in CCC , but its also hot. Its little noise. But also reach 100 degrees when playing and little cool when idle (52 degrees go down to 49 degrees)
Dont know why, im cannot explain. Im don`t know what to do now.