http://ltcmine.ru/ PPS 2.5% Fee

If Coinotron doesn't work for you. (I do better on ltcmine.ru)

I've had the same problem and the only thing that worked to stabilize was to reduce intensity.

Running 2x5770's I use "-I 16,17" with cgminer. The card handling the gui can't go above 16 and the second card can't go above 17 without instability. Since you have different GPU's your intensity numbers will vary. Check your System log in the Windows Event Viewer and you'll probably notice the driver stopped responding right before cgminer stopped hashing.

You might also want to try different Driver and Stream SDK combinations as that does matter. It's a pain in the ass to find the right combination, but might be worth it. Check other threads as this is discussed at length.

Even though I should probably be getting more like 390-400KH/s (Litecoin/scrypt), I'm comfortable with my 380KH/s right now as it runs stable for days without interaction and without heat issues.

Good luck!