While 85C may be acceptable to you, my GTX 280's in SLI never get in the 60's (air-cooling with aftermarket heatsinks). And (2) cards working together get hotter than a single card by itself so that's probably why it cleared up when using a single card. You should try swapping the cards so their in different slots and see if the problem comes back. If it does then the card in the top slot is going bad and you should try to RMA it. If both cards work good by themselves then you should look into improving your case ventilation and if that doesn't help then you should look into some better coolers or lowering the cards clock speeds.