Few questions:
Did you apply the Font fix for 8.6?
Exactly at what point is the finder crashing? Just as it reaches the desktop? After using it for..?
Are there any items in the startup folder?
Did you try starting with Apple only extensions? Or disabled extensions?
Have you tried working for a while off your system CD? Does it crash then?
Do you have an ADB mouse that you can use instead of the USB one?
Failing all of that, I would remove all RAM and put in one chip at a time to see if you still get that problem. I know it's a lot of work but sometimes it is one little thing that is causing the problem and it takes time to find it.
Be sure to press the CUDA button (little red one) after playing with the RAM.
Let's hope it is "a little thing" and not major hardware.