If I open a browser (in typing this) I saw the hash rate drop to 622khash/s, so I bumped the memory a little to 1440Mhz and the speed came back up into the 640khash/s range, which I'm happy with as long as I don't have any more lockups.
That is quite normal when your card "works". The best solution would be another GPU for your screen, so that your hashrate stays high.