You know, if your crashes are random, perhaps software isn't the issue. Check your CPU card, memory, connectors etc. Make sure your system isnt getting too hot or has a dodgy power supply.
Failing that, what 68040 card do you have? I don't think you should be using the PowerUP versions unless you have a phase5 card.
Check your OS install disks to see if you have the original version for 3.1.
If all else fails, try the mmu lib version.
