Calling all Athalon Xtreme GEEKS.

Problem: After 8-12 hours of screen saver activity (eg:overnight) Windows 98 underwater screensaver starts freezing for around 1 second every 5 seconds. My windows clock will lose about 40 min during this time And 3d game locks up. Clock resets on reboot. I have replaced everything except the pwrsupply, HDD, keyboard and mouse . Tried 3 differnt video cards,2 MOBs, 2 processors.RAM and freshbuilt it twice with 98. Installed cooling fans according to AMD and tempranges are at 40. Please help.
Biostar M7VKD 100/133 MO.NVIDIA 3dforceB32 plus. Simple 133sdram 2×128. Athalon 1.2 266FSB. WD 30 gig ata 100 with ata 66 cables.
300 watt Pwrsupply.