I've been having a weird problem this past week. Happened to me twice. One of my 2 video cards (GPU 0) jumps into Sha-256 mode or something, because hashrate is displayed in MH (around 470 MH/s). GPU 1 never did this. Good thing this happens during the day so I can restart miner fast. Any idea what might cause that jump?
Not knowing anything specific about GPU mining does not stop me from offereing an answer based on a life of electron herding:
Swap the two cards into the opposite sockets, then make logical conclusions based on the results. With any intermittent condition, after you intervene it may take a few days before the problem shows up again at all.
If your cards are connected through risers, you may need a second step of leaving the cards connected to the same sockets as you just swapped them to, but this time swapping the risers themselves.
I always like having things in pairs because it makes circuit-level diagnosis easier.