Jittering/Shaking display only in games

Hello everyone;

Just recently tonight I turned on the PC and opened up L4D to put some death unto zombies. To my displeasure, I received a jittering screen to the point of complete inclarity. My steps of troubleshooting follows:

1.) Ensured Vsync was enabled--check
2.) Ensured proper temperatures--check
3.) Factory clocked all my hardware--check
4.) Re-installed graphic and display drivers--check
5.) Recalculated wattage consumption to 650+/- using 1000w Ultra PSU--check
6.) Tried games in "windowed" mode--GLORY HALLELUJAH! Works great, but not in full screen.
7.) Tried various lower resolutions/settings to no avail.

I have owned a HP w2207 @ 60hz for a year and suspect but hope it isn't a monitor problem. Video/movies work fine, games=no go.
I have owned 8800GTS OC 512 in SLI for 6 months and rarely see temps over 45c. So, I know I haven't fried anything.
The driver version for the GPUs are 182.08 for Vista-64 that were installed day after release, Mar. 4.Have latest drivers for monitor from HP website.

So, ladies and gents I am stuck. Any suggestions or advice?

Your symptoms seem to me to be a psu problem. 3D puts added power stress on the vga cards. No doubt a 1000w psu should be sufficient, but is it possible that it is going bad?

Are the psu connections all secure?

Is there an additional mobo plug that might be needed to give more power to the pci-e slots?

How about trying things with each card by itself without SLI? Is it possible that one card is failing?

Do you still fail if you back off the 8800GTS OC?

You are both correct in the possibilities. I do not think it is PSU though. I tested all of the connections to ensure they're seated and have sufficeint output and it seems fine. I reset all of the memory cards in and out of the slots along with trading places with the GPUs. However, I did discover that when I disable SLI everything works fine... aside from the SLI of course. I tried each card independently to ensure they both work and they do. But when I enable SLI it is back to the flickering and ripping more-so than I have ever experienced. I did lower the OC that was factory clocked, but same thing there too.

I have done some searching around and found a few others with similar problems when SLI was new to the market in '05; they seemed to think it was Nvidia driver corruptions. I tried installing older versions of chipset/MB drivers and graphics, but still the same flickering unless in windowed mode or disabled SLI.

I suppose though it might be PSU; since I haven't actually taken that out and tried a spare yet.(don't really want to...pain in the ___). I have a 700w spare. I will give it a try and post back. If you come up with anything else I would very, very much be greatful. I am genuinely stumped here. Usually, problems are simple and staring me right in the face, but I am losing this battle it seems lol.

Thanks again.