I've noticed this as well. And it's not simply restarting the miner; I run mine with -r 240 so it should be restarting every 4 hours or GPU fail (I've not confirmed if this is happening in the logs yet FWIW). I got up this morning and hash rate had dropped and didn't come back until I rebooted. Bounced my machine and blamo... right back up to the 5130-5140 range.