This issue happened to me twice already out of the one month that i had this card. The GPU would suddenly drop its core clocks from 822mhz to 66mhz!!! i have no idea why it did that twice to me already. I thought this would probably be because of the temperature, but the GPU never really reaches 70c under load. It usually remains somewhere between 60-70c under heavy load. I'm getting very very close of doing an RMA for this card and i just want to get a second opinion before i do it.
For some reason the 550's have big issues with drivers. When I got my 550's at the time the driver was the 280.26 and I am still using that driver. I tried to update my drivers many many times.
Each time I would update my system would freak out and do weird stuff. It seems that what ever driver you started with is the one you will end with. I know a few other people who have the same problem.
It is not fact it is my opinion only that is a possibility that is the reason. If your driver is not the problem then don't take any chances and RMA it and don't get the same card get a GTX 460 or a 560 non Ti.
The 550's are the last generation cards and the worst gaming card that nVidia ever came out with. Well I hope you get it figured out, it sucks when you can't play good luck to you.