How long is the freeze?
Sounds to me like its more than 5 minutes to warrant a penalty.
I still think its the problem with graphics, since you mentioned that the screen froze during the occurrence. But you can also check your network ping during said freeze. If the ping spikes, then probably something is wrong with packet loss or some other app running in the background is hogging the network due to its high priority setting.
I'm just shooting random suggestions, but you can try adjusting your resolution setting next to see if the problem still happen in a lower resolution setting (or higher).
Do you have gaming mouse or gaming keyboard that might have bunch of customized functions? Perhaps the device driver is messing up. You can try lookup in Device Manager.

just example of problematic devices
My other method would be to pull out the graphic driver and try either the on-board graphic (if possible) and see if the problem lies in the VGA card. Of course this is a hassle to do.
You can try pop up your TASK MANAGER and keep it running in the background. When the error happens, try Alt+Tab and see which app is showing erratic values in the task manager.