At times.. cant click on specific instance of grouped app in task

An example of this..

Open up say 6 Windows Explorer instances.. they become grouped in the
taskbar.. if you want to click on one of the instances in the group.. you
simply cant.. the only way to get to them is to choose the cascade option...

Same thing with any other app.. like Internet Explorer etc..

The task bar will be working fine .. then suddenly this will happen out of
the blue.. no real reason why.. the only fix is to reboot the computer.

From what I can tell it only is occurring on x64 Vista..

Anyone else had this issue or found a way short of rebooting to fix it?

Thanks