Did you try different PSUs?
Yes, I tried on a different power supply - no changes. Still the voltage drops to 2.7V.
The power supply works fine on working motherboards.
Voltages measured on plugs with inserted graphics card, diagnostic card, RAM, processor, HDD and FDD and keyboard:
5.1V
-4.8V
12.1V
-12.0V
I also noticed that very rarely the 12V LED on the diagnostic card does not light up, but the computer boots. I have also noticed the rare cases when the monitor turns off after a short time.
I checked the jumpers again, replaced the cache from another motherboard - no changes.
I swapped the next two IC's. Nothing changes.
Sometimes it stops at C7, sometimes at E1, sometimes no indication on the diagnostic card. When it boots up normally, it sometimes gets into a restart loop with this message at the bottom.
Sometimes while saving BIOS settings, it freezes (only white bars at the top and bottom can be seen) displaying C7. It happens that the keyboard does not work fully in the BIOS, sometimes it does not work at all and the LED does not light up.
When error C7 won't go away, removing and reinserting the Dallas chip helps.
Usually, 5V is carried on an inner layer of the board, so I suggest trying to resolder all the +5V pins of the AT connector (best to remove most of the old solder first), and using deox-IT or a similar anti-corrosive agent on the AT power supply connector pins themself.
I replaced the entire power socket with another in better condition - no changes.
However, all these symptoms were present from the very beginning. So much work and no progress ..