I had systems that froze and I always ended up just replacing them. Time mucking around vs. buying a new board.

I found out my 2-port PS/2 KVM switch was causing some voltage irregularities on my retro PC motherboards with so many connects and disconnects. So the irregular freezes were probably due to the KVM switch and I was looking in the wrong place. The AGP issue was solved with a driver update, though in the last install it didn't. Maybe some order in which I installed drivers and updates. Anyway, I have been extensively rebuilding and retesting my two retro PCs, and I should give a better update in a week or so to wrap this topic up. My profound thanks to everyone who contributed in this thread, I have learnt a great deal in retro PC maintenance.