I had exactly the same problem with my Radeon 8500, eventually pinned it to the drivers. I got microsoft certified ones from the "Windows Update Catalog" where you select the product yourself and manually download the driver. The ones on this site are a lot smaller in size than the reference drivers from ATI. To get the link to the "Catalog" [catalogue... Americans!], go to windowsupdate.microsoft.com in Internet Explorer, click "Personalize Windows Update" and tick the "Display Windows Update Catalog" box.
Hope it works out for you.