That is a typical case of old connectors, i even see this on the A4000 desktops quite often now. replace the connectors on the psu and the one on the motherboard and you will probably get another 15+ years life.
As tenacious pointed out the connections develop resistance(usually they tarnish a bit which is the cause) and then start heating until stuff starts discoloring and melting.
Buy quality replacement connectors and your in business.I doubt the psu has gone bad.
-mech-
I'd been having trouble with they keyboard on my 4000T - dropping dead randomly, sending garbage input, and other symptoms that make it unusable. I replaced 2 of the capacitors within the keyboard, and that seemed to help, but it had no actual effect. My tests post-replacement were both false positives for success.
Today I started stripping down the machine, desperately trying to figure out what other component might be broken and interfering. I was getting nowhere, until I pulled the motherboard power leads on a whim. Yikes! The 5V rails are all fried. I'll bet money that this is the problem.
I installed this PSU in 2006. I thought that I would have gotten a little more life out of it (the original one lasted from manufacture in 1996 until my replacement). Is this damage to the leads indicative of something to do with my machine, or was this just a crappy PSU? It's 300W, driving a CSPPC, 2 hard drives, 2 CD drives, and 5 Zorro cards.
