Computer Issues!

Having serious problems with my computer...The iMac's screen has developed a curtain of green lines, making it impossible to use. It's been like that for the last few days but has, (partly,) cleared up this morning. How long it will be useable though is a moot point. I suspect that the problem will return when the computer is switched off or goes into sleep mode. We'll see but, if I'm absent from FB for a while, you'll know why. If it can't be repaired I'll have to buy a new iMac.