I have some mining hardware, including 8 x 5850, 1 x 5770 and 1 x 5750.
There are some people, who use same cards with GPU OC up to 950-975MHz with stock voltage (for example in this topic: https://bitcointalk.org/index.php?topic=34434.20
I have six Saphire 5850 Xtreme cards, max stable OC clocks are 875, 875, 850, 850, 850 and 800MHz. Two reference 5850s (powercolor and xfx) work stable with GPU@890MHz.
All the cards can't work at 900MHz or higher, I can set up core clock at 950MHz for example, but the PC freezes after i start phoenix and only reboot helps. If I set up 900Mhz clock, it may work some minutes, and then the driver throws an error and switches to default clocks.
I'm using windows 7 x64 on all my mining PCs.
I have tried different hardware combinations, different PSUs, no changes.
Should I try another OS (Win XP, Linux) for better OC?