Screen shows static "snow"

I recently installed os x 10.5.5 on my imac, upgrading from 10.4.11. After upgrading to 10.5.5 sometimes when I start my imac the screen is covered with "static" - similar to a TV screen when it's tuned to a station that doesn't exist. I can take a desktop icon and drag it around the screen "erasing" the static. Opening a large document that covers the screen also "erases" the static.
Has anyone else experienced this? I have a screen shot of the problem, is there a way to post it?

Thanks