On a few, but not all images, my screen goes black for about 5 seconds while I am using the adjustment brush in the Develop module.
I am running LR 4.4 on Windows 7, with I7 CPU and 8GB of RAM. No other apps are running; the resource monitor shows I am using very little CPU and about 50% of memory. I have an Intel HD Graphics 3000 card, and a Sony monitor running native resolution at 1920x1080.
I switched to a different image, of the same subject, and started making lots (about 50) adjustments with the brush, and I did not have any issues. I also exported the 2 images to a new catalog (to make sure I wasn't having a problem with my catalog being too large) and had the same experience.