Hi,
That sounds like tooltip timeout. E.g. hover mouse over poster name, see first a tooltip, then a thumbnail underneath; 5 seconds later the tooltip disappears but the thumbnail stays indefinitely. Perhaps you have something else hooked into that process?
Please test the issue in Clean boot mode to see if the issue still persists.
How to perform a clean boot
http://support.microsoft.com/kb/929135
If the issue doesnt appear, you can determine which one can be the cause by using dichotomy in MSconfig. Checking on half of Non-Microsoft service and restart, determining which half of the services cause the issue and repeating to check half of the problematic
half services.
Also, i find a similar thread, and the issue can be caused by a unknown process or malware. Please check the task manager once the issue happened to see if there is any unknown process was running.
For the malware, scan to remove it.