Weird hashrate drop

Hi

Have been experiencing some problems with my mining rig (mining scrypt altcoins) the last few days. Right now it only has a 6870 in it but i will be adding a couple of 280xs soon so i would like to sort out this problem before i get them.

So my problem is that some time after i launch the rig, usually about 1-3hours, my hashrate drops from 310kh/s to 250kh/s and i haven't been able to figure out why. Only restarting the rig makes it go back to 310kh/s. I have witnessed the drop in cgminer and it doesn't show any entries out of the ordinary.
I've tested several things: Different drivers, tested my internet connection, messed with most of the settings in cgminer, reinstalled windows 7 64-bit, tried out a different GPU etc. and still the problem persists.
It doesn't seem to be a problem with the temps either. I get 0 hardware errors and very few rejected shares. Before this problem came up the rig mined steadily 24/7 for a couple of weeks.

Here's my cgminer .bat file: --thread-concurrency 8192 --gpu-threads 1 --intensity 18 --worksize 64 --auto-fan --temp-target 68 --temp-cutoff 87

Could it still be hardware related? (Motherboard: MSI K9A2 CF-F AMD 790X, CPU: AMD Athlon 64 X2 6000+, RAM: 4Gb 800 Mhz ddr2)
Any and all ideas will be appreciated!

Thank you in advance for the replies!