System crashes when underclocked.

[taruntj5]

Hi guys, newbee in mining here,LTC to be exact, i'm using 2*280x's Gigabyte rev 1.0 version cards and getting about 1.2 Mhash/s. Now, i've read on various threads that if you change the core clock from 1100(default) to 1050, then one should get around 750 khash/s per card and I did that and it worked for a minute and the whole system just crashes. So, according to me, it's not a PSU issue coz i'm lowering the Core Clock not increasing. Anyway would like some help here??

System:
Motherboard - 990fxa-ud3
CPU - Fx-8320
PSU- Corsair Ax860i (Platinum)
Case- Fractal Design Define R4 (Using an additional 120mm sp120 fan on sidepanel other then two which already come with the case, all fans in pull)
Gpu1 is running at around 82 - 85C.
Gpu2 is running at around 73 -75C
 
And would love to hear about some tips I can use according to my system to get the best performance possible.??

Thanks

[HellDiverUK]

newbee in mining here,LTC to be exact,

You'll probably want to post of the Litecointalk forums, because this is bitcointalk.