strange Vista behavior that just started---screen blackouts with no relation to UAC

I have noticed a particularly strange behavior on my windows vista system (a Dell Optiplex GX270), that just started recently. Here it is. When I call up the start menu (not the classic style), and my mouse hovers in that region of the screen for about 10 seconds or so (whether I am looking through program lists or whatever), the whole screen will suddenly go dark for about five seconds (for no apparent reason) and then return to the previous state.

This is of course quite frustrating when I am going through the program list looking for a particular item. This blackout can occur a few times before I finally reach my target in the start menu. This seems to have nothing to do with UAC (which is turned on). This odd behavior started after I replaced my Dell wired keyboard and mouse with a wireless keyboard and mouse (Logitech).

In case the latter has something to do with this, I plugged in a wired USB mouse and removed the wireless mouse from the room. However, the same problem remained. (Windows update recently upgraded to Vista SP1, but this was a few weeks ago, so there does not seem to be any connection.)

I can find no help on the web, and I don't even know where to begin to troubleshoot this problem.

If anyone out there has any suggestions (short of wiping my disk clean and starting all over again), I would be most grateful.