iMac screen greys out

[ppnkg]

Hi, I'm having a problem with my early 2008 24' iMac. From time to time the screen is starting to white out (as if the brightness were increased and contrast were decreased without me doing anything) and becomes noticeably grainy. This is the third panel on my mac, I had to change it twice in the past, first because of dead pixels, then because it was greying out at parts. Any idea what this may be? I've noticed that when certain apps are open, particularly Numbers, this starts to happen after a few minutes. Any advice (except 'it's time to get a new computer!', which is probably true), I'd be grateful.