Who can say - why sometimes speed 1 of cards drops from 31M to 25M untill restarting miner?
Sometimes it is just Windows messing with one GPU (probably the active one, if you have a display or display-emulating "plug" in it). In such cases the hashrate recovers within few minutes.
Yes, this is the active GPU with display connected in it. But speed didn't restores in 5 minutes... It's too long. Average speed drops significunt for me. It's need to do something with that. Until that I'll use Claymore miner, in wich no such problem...