monitor image question

Can anyone tell me other than a defective monitor or video card what
else might cause my computer screen to collapse?
I have had the same computer and monitor for a couple of years now
(both bought brand new) and they have worked fine until recently. Now
after a few hours the screen seems to bend over on itself along the top
edge and sometimes even collapses to a bright horizontal line only a
few pixels wide across the center of the monitor. If I move my mouse
around I can see it moving in the squished image.
If I turn the monitor off and on again the picture does not restore.
If I reboot the computer it's ok again for a little while but after an
hour or so it will collapse.
So obviously a defective monitor right? Wrong. I have connected it to
another computer and it works fine. (The other computer is a laptop but
has the same screen resolution)
So obviously a defective video card right? Wrong. I went and bought a
brand new video card for my system and the problem still occured.
Anyone got a clue what else might be going on here? My only guesses
left are either the powerbar / power cable or maybe the motherboard? I
doubt it's the motherboard because the video card is what drives the
monitor not the motherboard, right?
In case people need machine specs:
Intel Pentium 4 2.4GHz with 1024 megs of RAM running Windows XP Pro
(SP2)
Monitor is a NEC Multisync FE991sb
Video card(s) are both nVidia GE Force, the original being a FX 5600
and the new one being a 5500 OC with both the latest drivers and even
with the drivers packed with both cards.

I'm out of troubleshooting ideas at this stage.