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

This has been a common problem across the line with the new Nvidia cards, but I am very surprised to see it on the OPs Mac as it is older and therefore probably doesn't have the Nvidia chipset.
See this thread for the fury:
http://discussions.apple.com/thread.jspa?threadID=1924473
There are many others too, the snow is common. Hopefully they will fix it in a software update since basically every new Mac has the Nvidia chipsets in it.
P.S. I have not had it on my 17" MBP with either the 9400M or 9600M enabled, however, I have seen it on an older Mac Mini, so I think it has something to do with software.

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