Have you looked at the P8/P9 powerconnectors on the motherboard. Are the contacts on the plug slightly dark or did the plastic of the plug even turn brown near the 5V line ? I have the 3rd powersupply in my A4000T now, as the 5V lines to the motherboard regularely burn away over time.
I admit that I have a lot more stuff in my A4kT that drwas power from the poor 200W PSU (CSPPC, CVisionPPC, 4 HDs, 5 Zorrocards etc.) but I don`t let it run 24/7, so there is a good chance that you PSU quit after 11 years while mine did after just 3.