First post, by Moogle!
Brought it inside and went about cleaning it up. It had an Asus P5S-VM with k6-2 380 (95 x 4) and 64MB ram. I have since updated the BIOS from the limited HP supplied Phoenix to the Asus supplied Award, v1005, which gave me more options and settings. (Getting the new BIOS written turned out to be an issue itself). The Motherboard has the (in)famous Sis 530, with 'support' for the 133MHz FSB.
I have been experimenting with various configurations using the K6-III 450 and memtest.
133 x 3.5 (466) with motherboard cache: Errors
133 x 3 (400) with motherboard cache: Errors
124 x 4 (496) With Motherboard cache: No errors
133 x 3.5 (466 without motherboard cache: No errors
I played with this for awhile, trying a few things like changing voltages, changing ram modules (all used were 7/7.5 ns (133mhz) modules), putting a better heatsink on the chipset, and installing an external video card instead of using the onboard video. None of these helped. So I started looking at the board itself. I noticed that while the motherboard cache is rated at 5ns (200 mhz), the cache chip's tag ram is only rated for 8ns (124Mhz), and I believe this is why the board isn't stable with the cache turned on at 133Mhz. (Incidentally, I have seen older style 286/386 simms with tag chips that are slower than the main ram chips, so if you have problems, see if that is the cause.)
Any thoughts or questions, Vogons?