Monitor picture collapsing

I'm asking this here as I've heard of it happening on all sorts of systems before, as though it's a common fault and possibly with a common cause... I've got a mono fixed-frequency display that'll often power up and display fine for a few seconds, after which the vertical collapses and the horizontal shrinks slightly, such that the picture vanishes to a single (squashed) horizontal line in the middle of the screen. The "picture" goes rather dim at the same time. I would have thought that was a power supply problem of some description - except that some initial checks show that the expected single +12V supply for the display is at +11.8V, so only slightly down on what it should be. Hmm, I suppose I could try feeding +12V in from a bench supply and rule that one out... cheers Jules