As has been stated it's most likely an unstable OverClock ... but there is a chance it's a combination of the OC and the AMD CCC drivers, I've read some people that had these issues.
Here's what I would try in this order ...
- How did you "UNLOCK" the HD-6950 ? is it a Dual Bios Card ? or did you have to FLASH it ? and did you FLASH it using the proper BIOS for your card ? or did you maybe use a HD-6970 Bios from another card, as that often can be part of the problem, you can't just interchange BIOS files like lego's ... you need the right BIOS/Firmware to work with your particular Hardware. So check this 1st.
- Next, try the NEW AMD Catalyst 11.12 drivers, they have been getting GREAT reviews and result from many people so far and have fixed many issues for folks.
- Also, what HD-6950 do you have ? most likely it's stock voltage is 1.1v and to get that OC you might need to over volt it some, you should be OK to go up to 1.25v, but experiment in small incremental increases until you can retain stability and then don't take the voltage any higher than you need to.
- Lastly, if you don't want to mess around with and/or don't know how to increase the GPU voltage, then you'll have to back your OC down a bit to get stable. Also remember OC Core 1st and Mem 2nd, too many people just try to bump up everything all at once and then get upset when it's unstable. Get the Core Clocks as high as you can 1st, and then work on the Memory clocks, and once you have reached some stability back the Memory clocks down by 10 MHz or so to play it safe.