SN85G4 V2: use generic PSU for diagnosis?

Hi,
My SN85G4V2 has flaked out this week. It still sorta works, but I think
it's having power supply problems.
In trying to troubleshoot this, I wanted to take a spare desktop 300W
ATX power supply and hook it up to the XPC. I did this, and while the
system seems to have power, it won't turn on. And yes I hooked up the
extra 4-pin 12v CPU power connector.
Does Shuttle do something non-standard with the ATX connector? Ie,
extra power sense or on/off lines or something like that?
Has anyone else had to troubleshoot PSU problems on their XPC, and how
did you do it?

My configuration, in case anyone's curious...
AMD Athlon64 3200+
1GB Kingmax PC3200
ATI AIW Radeon 9600XT
Buslink USB2.0/1394/SATA PCI card (for maximum expansion!)
WD 200GB SATA drive
Lite-On XJ-HD166S 16x DVD-ROM

thanks,
Jonathan