P!!! and Prime95 no dice?

I'm currrently upgrading my inlaws' system. It's a P3-933 with 2x64MB PC133 CL3 on an Asus CUV4X-S board. Now I threw out the 8MB Ati card, cheap soundcard and the crap RAM, installed a GF4-MX440, 256MB PC133 CL222, SBLive!1024 and a NIC. I OCed it mildly to 1000Mhz (143FSB). Now before I do a clean install of Win98SE I wanna make sure this thing is 100% by running Prime95, BUT: when i start the torture test it gets busy on the first iteration but nothing happens anymore. The system doesn't hang and I can move the Prime window and access the rest of the system and such. I let it run like this for over an hour, no errors but nothing more happens either, WTF?

I've had that same problem when Prime was using more than 85% or so of my memory for a torture test. I'm curious to know what would cause this.