IMac :: 24" Recently The Screen Has Began To Flicker?

i bought a used 24" aluminum imac (2008 model) that was about 6 months old, but recently the screen has began to flicker...usually from a half to a full second every few hours or so. it sometimes will flicker if the mac is nudged on either side, but this doesnt always happen, even when i TRY to initiate it. its not really major, but im just worried that the whole display will crap out soon. ive found threads about screen flickering with macbooks and led displays, but nothing for imacs yet. im not sure if this matters, but the person i bought it from opened it up and replaced the hard drive. another contributing factor may be that i constantly run torrents and my mac sometimes stays on for days at a time. it doesnt seem to overheat and ive never had this problem with my other imac (2007 17" white). when im not using it, i keep the disks active, but put the monitor to sleep to prevent burn in. im assuming that since this guy replaced the HD, this would void any warranty, so i think im pretty screwed as far as thats concerned unless i pay out of my own pocket.