No updates, I switched I/Os with keiichi and it hasn't frozen on me just yet. Next time we have a long session, I'll keep an eye out to see if it freezes.
My IO froze even without linking to another machine. Second player was controlled via maple adapter from the filter board. And it would only freeze between games. Never during. It’s a software issue.
It's random. It also freezes via GDRom. Keiichi331 tested it just to rule out all possibilities. It will freeze both for CF card reader and Netboot too. I even tried different roms via net boot. Lock, unlocked, PAL colour hacked.