iMac freezes on Apple screen loading circle

[duky]

Anyone having/had this issue with their iMac? Basically every once in a while I come back to the computer and it's semi-frozen--that is, everything functions but I can't switch between Spaces, I can't use the Corners to do anything, it's like semi-functional. If I try logging out it first says it can't quit applications then when I quit them manually it ends up just freezing right before the Finder closes so everything else on the screen is gone. Then when I manually shut down and restart the computer freezes on the grey loading screen after the loading spinner stops spinning. The only way around it I've found is to shut down, unplug the iMac, plug it back in later then it usually works. Any help would be appreciated? Taking it to the Apple Store will be a huge hassle