i bought an evga gtx 550 ti sc 3 days ago. replaced my old radeon hd 4670. so i uninstalled the amd drivers normally (control panel) and installed lastest nvidia drivers (301.42). everything was fine, it played games pretty good. using evga precision x to control fan speed. no overheating issues (always watching temp), have good airflow, good cable management, not many dust in the case. not so much hardcore gaming these 3 days. so today i was playing crysis 2 (less than an hour playing it) at high settings at 1080p (45-60 fps constant) and suddenly the fps dropped to 4-8. the temp was around 63C (which i think is normal under load). i thought it was the game or something, so i lowered the resolution but i still had the issue. exit game, then open another game, same issue choppy and laggy 4-8 fps. temps are still good. so i opened evga presicion x and i noticed that the gpu clock went from 400 mhz (i believe) down to 66 mhz and the voltage went from 965mv (or something like that) to 1125mv. i didnt had any issue before like glitches, bsod, black screens, no response or anything else. i havent restarted yet the pc, cause my hd 4670 got artifacts and the first thing i did was restart and it never booted again. i hope this has a solution, thanks.
edit: forgot to mention that i havent messed with any setting or tried to overclock cause i dont know anything about it...
 
Asus M4A87TD/USB3
AMD Athlon II X2 265 3.30 GHz
G.Skill ECO DDR3-1333 2x2GB
WD Caviar Black 500GB AALS
Windows 7 Ultimate 64-bit
EVGA GTX 550 Ti SC 1GB GDDR5
Ultra LSP 550w PSU

Hmm seems like its down clocking it self.  This usually happens if your overclocking to much or its not cooled enough but since you say you haven't touched any of that then im not sure. 
 
Maybe its just a faulty card>  I would contact EVGA and see if you can set up a RMA on the card.  Maybe someone else will chime in on this.

thanks for the reply but damn i know if i have to RMA it will be a pain in the ass cause i live in puerto rico (shipping/time)...

I am having the same issue here. Playing Diablo 3 and all of a sudden the game drops in FPS.
 
Normally when I tab out of the game the Core Clock drops to between 50-400MHz, depending on the need of the card.
 
When this happens it appears that the core clock gets stuck @ 202MHz (Overlay shows 203MHz, so I assume its somewhere between 202-203). Temp is under 50C and Fan speed is @ 30% every time it does this. Voltage though doesn't change.
 
Changing games doesn't solve the issue and it lingers into the new game with low FPS. and Core Clock is still stuck @ 202-203
 
Only way I can fix this is by doing a system reboot or my increasing the core clock in Precision and then lowering it back.

I've run into the same issue. It only happened when I updated to the 301.42 drivers.
 
I had to downgrade back to 296.10 to get it to run smoothly again. Something with the 301.x and 304.x drivers that don't like the 500 series.