Flickering 24" iMac

I have a 2008 24" iMac (model 8,1) I bought it new in 2010 from MacConnection. This machine has been wonderful, however in the last week or so the screen seems to "flicker" upon waking from sleep (I rarely shut the iMac down.) The flickering lasts for a minute or two and is sometimes worse than others. Occassinaly the screen will distort and go black for a second or two, other times it just seems to flash. As I have said this only lasts for a minute or so upon waking the iMac. I can work all day without a repeat.

Has anyone else experienced this? Does it sound like a hardware problem, perhaps the video card is giving out?

I welcome all comments.