85c idle temp??? thats very high
i have the ATI HD 6870 and mine also heats up to 90c after about 2 hours of play, and then i start to lag. But when i shut the game down for a bit my idle temps are more in the 50-60c range. 85c idle is far from normal unless your pc is sitting out in the sun when you play.
Dust that thing, and if thats not enough you may need to explore some other cooling options for your case.
Also depending on your card you could find a program to force the gpu fan to run faster….. but cleaning out the dust is the first priority