Today was the first time since my previous post that I had the problem again.
The problem happened about an hour before I got home. When I got home I noticed it was more quiet than "normally". All gpu fans were running @ low rpm's.
Both cgminer 3.7.2 & cgwatcher 1.3.4.6 were frozen with empty windows.
When I closed the cgminer window, cgwatcher unfroze and it restarted cgminer, after which the cgwatcher overheat protection kicked in with fans @ 100%, as my gpu's were @ 90 Celsius.
Haven't rebooted this miner since it happened. But everything appears to be normal.
If you haven't already, please download the latest version (1.3.5.2) as it should have some additional handling for a blank miner window.
I've been using 1.3.5.2 for a couple of days. Just now I had another blank window problem.