weird menu problem

I've been having this strange menu problem since I got my new Imac, the menu items (folders and icons) appear as dashes (actually they're just the bottoms of the normal icons) and the "expand" arrows disappear entirely. My sessions usually start out ok and then the menus go flonky (happens to the green dots on the mini-map too)

Here is a link to an example inventory image...

http://farm3.static.flickr.com/2237/2304614447_e7cd386a0b_o.jpg

I'm using an intel Imac, with the ATI RadeonHD2600 graphics, and OS 10.5.2

Anyone else having this problem? it's really starting to annoy me.

Yes!!! This usually doesn't happen till I've been on for several hours. All I know to do is to relog, it solves the problem for a while.

I have the same iMac, same graphics card.