Is this underclocking while idle?

So i have a 550ti, my old PSU gave its last breath, so i bought a corsair 500 watt, hooked it up, and now in precision x my card seems to downclock at idle which it never used to. Its at its normal 900 mhz when x opens but then it downclocks to 50, and goes up to 450 when im doing something like opening a lot of tabs at once or video editing. Is this normal? It never used to drop or go above 900 at all ever. So this is new to me. Ive googled endlessly and seem to have found my answer in these forums actually, but could someone verify what this user has said. http://www.tomshardware.com/forum/348326-33-clock-speed...