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!