The system did not shutdown like it had been until I installed a PCI NIC card, then it would shut down almost immediately after the inital motherboard flash screen.
What do you mean by this? Was this part of the original problem?
Also, I was going over everything that you had done - are you plugged into a UPS? Have you tried plugging directly into the wall outlet?
I guess that I would strip the computer down to: video, one stick of RAM, 1 harddrive; nothing connected except monitor, keyboard and mouse (preferably PS2 for the latter 2) and see if it's okay. If not, switch RAM modules; also switch video if you have on board and a separate video card. (One change at a time.) If stable, replace the RAM stick with the next one (but just run one module at a time). If stable, add the second RAM module; then add back harddrives, one at a time; then add back CD-ROM; then add back any PCI cards. Allow enough time between changes to allow it to shutdown. This is poorly presented but I hope that you get the idea!
You've mentioned the motherboard as a suspect twice. Certainly examine it closely for bulging capacitors, signs of overheating, physical damage, etc. If the above doesn't pinpoint another cause, the motherboard may have gone bad.
