I am running Windows 8 Professional (64-bit) and often keep Task Manager running and minimised
I always have had it running, minimised, since way back with Windows 2000. I don't need it, it's just a comfort blanket: useful when things start going slow, but normally just a reassuring little bar chart in the notification tray.
However, I've noticed recently that it is often paused. I can set its updating speed back to Normal, but during the time it has been paused, the data has not been recorded - and invariably, when I switch to Task Manager it's because I wonder what's going on - and then I can't see, because it's not running.
I don't remember even seeing a Pause feature before Windows 8, but certainly never saw it automatically pausing. I'm absolutely certainly not setting it to pause myself using the mouse, but wonder if there is some event or keyboard shortcut that triggers it?
On the other hand, you might want to upgrade to 8.1 anyway, and that might fix that ;-).