Intel Imac display goes to sleep spontaneously

my Imac, running ML, the display appears to go to sleep spontaneously, and quite often (increasingly so). It doesn't seem to be actually going to sleep - I say this because the only way to revive it, ofther than hard restart, is to put the machine or the monitor to sleep and then wake it back up. The HD is not effected - it still spins when the display goes black, and I can use the keayboard shortcut for 'display sleep' to revive it. Will zapping NVRAM fix this? If not, anyone know what will?