My CF 4870's dont like certain Overclocks

[Born_2_Kill_83]

I have an issue thats developed with my 2 4870's.

If I run the highest CCC overclock possible (790-1100) I run into problems, while benching 3D Mark 06 Ill get flashing corrupt graphics in the valley test for a short period and if I play a DVD in media player the computer will actually crash.
However if I use stock clock speeds or higher 810-1150 (which is totally stable in every test) I dont have any issues at all.
This has been the case with both 8.6 and 8.7 drivers.

Should I be concerned that my GPU's have a problem, or is it simply an OC they dont like??

 

[FreeStream]

If its ok at a higher clock it could just be thats a clock they just don't like.

Keeping them as cool as poss tho would help