Xserve shuts itself off and boots at will
The original G4 Xserve was prone to structural 'sagging' after spending a while in a rack. I'm not aware of any specific fix for this problem, which causes random kernel panics / shut-downs / other weirdness; later model XServes were not affected (not sure when exactly, but I've never seen this on any intel xserves, while I did see it on a significant number of g4 serves).

http://tidbits.com/article/11735

Sorry for the bummer news :/ If you can get your hands on a late-model intel xserve, they still hold up pretty well. I use several of them on a regular basis.