Which miner, which flags, which driver, which SDK?

The golden settings for a 5850 are usually 10.11 driver, 2.1 SDK, phoenix miner, VECTORS BFI_INT AGGRESSION=13 FASTLOOP=false WORKSIZE=256

use phatk if you are on newest drivers, poclbm if on 2.1

Aggression can be dialed down if you use the computer for anything else.  Worksize should be tested at 128 and 256.

Get afterburner and bring your memory down to 300MHz

Edit:  If you are using guiminer try -v -w128 in the extra flags

Edit 2:  I would recommend running Driver Sweeper, then reinstalling the newest drivers, but only the driver and the sdk.  Don't install CCC or overdrive or any of the extras.