I had a Shuttle SV25 and had similar sporadic stability issues until it just refused to boot at all. I will point you straight at the dreaded faulty capacitor problem.

I would completely disassemble the machine and take out the FV25 motherboard to have a close look at the capacitors. Some are likely to be a little swollen with raised tops - these are bad. Badcaps.net have more info.

Before replacing capacitors:


First boot after replacing capcitors:


I passed on that machine to my cousin and it is still working, even if it is ridiculously noisy by modern standards. If I remember it has a 1.26GHz PIII Tualatin processor in it with 1GB RAM, which makes it pretty nippy too.

Good luck!