I had the same issue with my xfx gtx 260's here is the fix that I found.
1. IN RIVATUNER CLICK ON THE POWER USER TAB
2. CLICK ON + RIVATUNER \ NVIDIA \ OVERCLOCKING
3. CHECK EnablePerfLevelForcing , VALUE = 1
4. MAIN TAB , DRIVER SETTINGS , OVERCLOCKING
5. IN DROP DOWN BOX = FORCE CONSTANT PERFORMANCE LEVEL!
6. APPLY AND IT WILL ASK YOU TO REBOOT , DO SO!
Hopefully this will help you out seemed to fix the issue for me.