I think I had this problem back when I was still refurbishing my cube, and I believe the problem was bad ram.
Do you have multiple sticks? If so, try one at a time to isolate a possible bad one.
Here is a link to the service manual for the cube:
http://tim.id.au/laptops/apple/powermac/g4cube.pdf
It has troubleshooting stuff at the end, and has been the best resource for me when working on my cube.