Since two laptops do it, and they are from different vendors, that points the finger pretty squarely at some software program you installed on both systems.
So, to figure out which one it is, you need to strip one system down to the bare essentials, and then start adding things back in one by one until the problems surface. Probably the quickest way to do this would be to format one system and then add back one program at a time, per day, until you find the problematic one. Then you either get rid of it, find an alternative, or hope there's an update to fix that problem.
I have access to 2 laptops running Vista. One laptop is a Dell Inspiron I6400 Intel dual core, 2 gigs of ramm, 150 gig hard drive. Laptop #2 is a Sony Visio with 512 gig of ramm, 120 gig hard drive, and Intel dual processer. Both laptops within 10 - 30 minutes of start up will momentarily go blank not just once but several times. After a half hour or so no problems. It's more of a nuisance than anything, but when I am working on a word document it's frustating because all of my work is lost.