iMac screen pixalated & static at startup

about every 3-5 times I turn on one of our iMacs at work the screen gets a pixelated sporadic static that goes across in 2-3 vertical lines about 1/2" thick and varying in location often times jumping around. Once it gets to the login screen it is very pixelated and if you go into display settings it the area to change the resolution is blank. If you restart the computer 9 times out of 10 it'll startup normal and on the off chance it doesn't a second restart will resolve the issue. It started this right out of the box the very first startup with no external displays connected. Ive had it for about 2 years and when I contacted apple support they had no idea and suggested taking it to an apple store. Since we use this computer often for our media production, taking it in was more trouble than it was worth. Any ideas of what the issue can be? Im guessing its not something that can be resolved without a taking it in but I figured it was worth a shot.