Okay, since you hadn't statted trying a different monitor in your question, that is why I asked that question first.
After looking at the manual for your motherboard, I have a new idea.
I have had numerous 'strange' problems with motherboards with the VIA KT133 chipset.
The 'fix' has been to manually set the CPU FSB and clock multiplier in BIOS settings.
If you have a 200 MHz FSB CPU, set the FSB to 100MHz, and the multiplier as needed.
If you have a 266MHz FSB CPU, set the FSB to 133 MHz, and the multiplier as needed.
If you have PC133 SDRAM, set the RAM speed so that you end up at 133MHz.
A good link for information on VIA based motherboards is www.viahardware.com
Pauls FAQs are very helpful.
Another thing, how much RAM do you have?
While it should not affect things while in BIOS settings, Windows can generate some 'strange' bugs if you have over 511MB of RAM installed.
I limit ALL of my 9X boxes to 384MB (3X 128MB).
One last thought, have you swapped thesystem power supply?
I have noticed a rash of power supplies that are 'peak' rated, and not continuously rated.
I have a 350 Watt peak rated supply, where the continuous power is under 250 Watts.
Another last thought, look at the motherboard around the large capacitors. There was a problem a while back where one of the capacitor manufactures made a bad lot of caps.
Parts from this lot fail by leaking the electrolyte. If the area around the base of the caps looks damp or has crystaline deposits, the caps may be bad.
Chas