Nonsense, there is nothing wrong with the design or function of the original 4000 psu's....
I don't disagree with you. When the A4000 was designed it was a standard, if low cost PSU for the time. It's not like Commodore were breaking new ground with amazing PSU tech.
And I agree with the recapping of the original PSU and while you're at it, putting in a quieter fan.
I just found it a lot easier to put in a modern, manufactured in this century, high quality PSU. Only a few OEMs make a SFX and/or TFX PSU and they are reputable brands and the PSUs are generally of the 80+ certified variety, and run extremely quiet.
Whilst it is true that ATX PSUs do place more focus on 12V, the 80+ have solid 5V performance.
You clearly agree that good clean power is important. You can get it by improving the original A4000 PSU or by getting a brand spanking new SFF PSU.
For the record, the PSU is fine...
I wasn't implying that your PSU was dead. What on first glance is a regular running PSU can still be a culprit for many things that go wrong with one's rig.
I'm not saying you must buy a new PSU. But I would at the very least recap it as per mechy's inclination.