Sorry, but I don't know the answer to your problem, but the reason I've found your post, and this forum, is because I have exactly the same problems you describe with my son's ST61G4 system.
I have tried a different processor, different memory, removing the graphics card, and the wireless g card, disconnecting everything, resetting the CMOS, removing the battery (didn't have a spare - but looks like you've covered that base!)
Damned annoying, and I've already replaced the system, but would love to get this little box up and running again.
So hopefully someone will suggest something - again, all the CAPs
appear fine.
Let's wish ourselves luck!