the pictures look good and it looks healthy
in such a case i would recommend try another wall socket; temporary test and a surge protector.
re inserting ram and addon cards gently can help.
a test of ram and hard disk can help also.

Stability problems can also come from connectors, loose connectors, slightly corroded connectors, halfway inserted connectors, damaged connectors, overheated connectors, oxidized connectors,...
if you think it's a voltage problem, yet the psu voltages all measure ok, as in, you measure the voltages directly on the psu connector and they're all ok.
Then you should try to measure the voltages at some of the furthest points away from the psu.
It will be a bit tricky to find some good points to measure but, looking up some datasheets for chips can be really helpful in such cases.
The main voltage to check on older (AT or from the same era) computers would be the 5v line.
It should measure close to 5v all over the board. If it is like 4,6v, I would call it suspect, especially if you measure 5v at the psu. Anything lower is definitely going to be a problem.

oh and don't just pick the last (e)isa slot to measure the voltages, that's too easy and not indicative enough.

Hi,

Thanks for pitching in and for your proposals.

It seems I have found the problem, and it was not related with the Power Supply...

... it was the CPU.

I have procured an Intel 486DX2/66 to replace the Cyrix cx486 DX2 - 66 I had in place and all of a sudden, the system became stable.
I've been abusing it for 3 days and it's rock solid.

I never suspected of the CPU because in 30 year of working with PCs, this is the first CPU that has ever failed me, especially in such a weird way.

Well, living and learning.

Thanks again for trying to help. This is what makes this community great.

Cheers,