Scrypt mining - what is wrong?

[Remember remember the 5th of November]

I have Windows 7 x64, and a 5850 and 5870. The 5870 is the PRIMARY card on which I game and the one that has the problem, the 5850 is the secondary card which usually mines and has 0 problems, knock on wood..

The problem with my 5870 is that sometimes when I enable the GPU on cgminer 3.7.2, it makes moving windows, scrolling extremely laggy, but at the same time I see the card mining with at most 16-30kh/s. This is at intensity 12. When the problem goes away by itself, I re-enable the GPU and it works just fine, reaching 310kh/s at same intensity but NO LAG.

One thing I notice is, the GPU VRAM usage does not go up when the problem is there, when it's gone and I try to mine again with the 5870, it's VRAM usage goes up. So I think what happens is, the GPU's memory is not available for mining at times. I have the latest AMD drivers and no OC.

[LtPaxIV]

hah.got the same problem sometimes. 5850 secondary 7850 primary.the 7850 makes sometimes same wierd thing.i figured when i restart my computer and had disabled second desktop on the 5850 the 7850 isnt behaving as it should,so i enable the 2. desktop on the 5850 restart the 7850 and it works, after that i can switch off the 2. desktop on the 5850 and everything works normal.

what this is and why i dont have a clue because its not always like that