Monitor Flashes On/off - Multiple Pc's!

Hello

I hope someone here can enlighten me on a very strange problem which i have never experienced before. I have just completed a new office setup and there are 18 workstations used. The machines are old, cheap "Toshiba Equium" desktop PC's, each powered by either 600 or 800 MHz Intel Celeron CPU's, each one equipped with 2. 128 MB of SD-RAM and a 30 GB hard disk. Graphic, LAN, sound etc is on-board, all the machines need to do is handle office documents (word mostly) and internet surfing with Firefox. OS on each one is XP Professional (genuine) OEM.

Almost all of the machines have spent the last year or so in storage, i.e. have not been used. Upon setting up the office i found several of them needed to have the RAM re-seated, two were just "dead" (turned on but no reaction at all apart from the "Power" light and the CPU fan spinning) and two were blinking their monitors on-and-off, but working normally apart from that.

So i started changing and swapping parts around, but i could not resolve that "blinking monitor" problem on those two. My conclusion was that it had to be something related to the mainboard.

That was last week. As of Friday i had two more of those machines "doing the blinking", suddenly, after being active for a couple of days without problems. Again, swapping RAM, CPU, HDD and monitors doesn't resolve it, so it must be the mainboard (the mentioned parts is about all that can be swapped on those things).

And today in the morning i had a FIFTH machine doing it! And now it's driving me nuts. What on earth can cause a computer to suddenly start flashing it's video on-and-off with everything else functioning perfectly normal?? There is no error message or BSOD, Windows doesn't hang or reboot - it's only the monitor that goes on-and-off.

And it's not the actual monitors either because that same monitor with another computer attached works fine, while that same computer on another monitor blinks on-and-off as well, even with a CRT (all monitors at that office are brand-new 14" LCD's except for one).

I googled for a solution but found nothing that applies here - screen refresh frequency: Is set to 60 with all of them, in fact can't be changed. VGA card - none, it's on-board. BIOS battery - tried (removed, "clear CMOS") - no change. Uninstall and re-install VGA driver - no change. And it's not a Windows problem either because it happens from the moment the machine is switched ON, i.e. during the POST and pre-Windows-boot sequences.

Another thing is that during the machine "sitting idle" it only blinks about once every two minutes or so, but when there is activity (starting a program or trying to work something) it blinks every second, making it completely impossible to use them.

What could it be? Is it possible that old computers just die like that, one after another?

With kind regards......

Thanh