It's what the GPU does to save power when you aren't in need of full 3D mode...it's normal downclocking.
As long as it doesn't happen while gaming or folding, you're good

Also, if you are folding and it does that you may have an unstable overclock. If it is dropping while folding, it may be that your video drivers failed and were recovered by windows. It happened to me on a couple different 400 series cards, and I had to lower GPU clocks to stabilize.
 
EDIT: Hopefully you have the fan speed set on auto or a fan profile set. Folding with the fan speed set manually to less than 100% can cause overheating, which can also lead to driver failure/windows recovering the video drivers.

Arctic. Light

thank you ^-^ lol just needed to lower my clocks and i was good 

 
You're most welcome 

SirJamesDDJ

BR nominated for Arestavo.
Well done!

Thank you, Sir! (JamesDDJ) 

Afterburner

AB Was here...

Another BR Cherry pooped... MUhahahahahahaha

Eeewwww! AB is pooping cherries now.

I have this same problem, does it when folding or gaming and started after I installed the 260.89 WHQL'd driver. Went back to a older driver (used driver sweeper) and still the same problem.
 
I noticed if say I start gaming or doing something that uses it's full power (folding) within a minute or 2 of rebooting/loading windows it will run full speed no problem for that program but if I then quit that program and switch to something else it will go down to 405.
When it is stuck at 405MHz it does not even downclock when idle to 50Mhz like it used to.
 
Running factory set speeds on a 480 FTW, even underclocked it and same thing.  Manually overclocking or underclocking does nothing, just locked at 405MHz.
Was certainly something with that driver (260.89 that messed it up but can't figure out what or how to fix it yet.
 
Oh and temps are good, 37C at idle and up to 42C load with my water cooling.
 

Actually got it working right again, cleaned out the 258.96, which still had the 405MHz lock, that I went back to after the 260.89 problems. Well cleaned those out and went back to 197.75 and everything started working correctly. Then went back up to 258.96 which was the last confirmed working driver for me and all is well now.
 
I personally think the whole thing went bad when I elected not to install the HD audio and 3d drivers in the 260.89, guess Nvidia took offense to my not installing their bloatware. LOL
 
Do I dare try the 260.99.