MacBook Screen Flickers, Grays Out

Occasionally when using my Macboook (2008, Aluminum) I notices that the screen will flicker a bit and then "gray out" slowly. The mouse/ trackpad stops respoding as this happens.

To describe more fully: The screen never seems to go dark, it just sort of "pulses" and I see everything start to fade to gray almost as though it were shutting down. But you end up with a gray screen. A restart seems to solve it, but I am trying to see if this is a hardware issue, software issue, or if I've been infected with some really weird malware/ virus.

I am running the latest (AFAIK) version of Mountain Lion. (10.8.3)

Software OS X 10.8.3 (12D78)

Graphics NVIDIA GeForce 9400M 256 MB

13-inch, Aluminum, Late 2008

Processor 2 GHz Intel Core 2 Duo

Memory 4 GB 1067 MHz DDR3

I jst took the Mac in for repair a few months back (December) and Apple basically replaced the entire guts of the machine. So I can't see wha tthis might be.

Any help out there? I have seen some similar problems mentioned (or at least, I think they are similar) in some years-old threads, but they don't seem to describe what I am seeing.