GPU downclock???!

Hi, I'm currently having an issue with my GTX480 SLi. The gpu clock, for some odd reasons, downclocks and that is during gaming. When idling it goes to 50mhz which is comprehensible since im not giving any load to the gpus... however I don't understand why they downclock from 700mhz to 202mhz while playing ... Also, one card has a voltage of 1050mv and the other card has 1038mv: why is there a difference?

here are my specs if needed :

- AMD Phenom II x4 965 BE oc'd @ 4GHz (stable)
- EVGA GTX 480 SLI
- MSI NF750-G55
- 8GB DDR3 1333MHz G-Skill ram
- 1000W cooler master silent pro PSU

and the game im playing it on is Mirror's Edge. Note that it happened on other games such as Batman Arkham City etc...

Thanks for your time!