My Mini was crashing every eight hours or so. It took me a while to figure out that I needed to boot to the "original install disk" in order to access the diagnostic utility. Apple does not put the hardware diag utility on the retail Snow Leopard install disk so you need to use disk one of the pair of gray disks that came in the computer's retail box.
The diag util gave me an error code and google found posts recommending I remove the DIMMs and put them back in. These DIMMs have been in the machine for almost a year, so they would have failed long ago if they were bad. I took them out and snapped them back in, reversing their positions. Since then, the Mini hasn't crashed.
I also swapped in a new, 160GB SATA drive which spins at 7,200 rpm to replace the stock, 60GB 5,400 rpm drive. The new drive has a larger cache and supports NCQ, which is also supported by the SATA controller. This box seems a good deal more responsive now.