Bad +12v with Herolchi 400w LDT

PSU: Herolchi 400W LDT
Problem: 12v falling to 10.9v

As above, sometimes the 12v is falling to 10.9 - 11.2v after some time. The only solution is to take off the plug from the mobo ATX socket, and re-insert. I wonder if this is a mobo problem. If the PSU is faulty, then the problem should not go away by simply doing that, right ?

Could be a mobo issue... have you taken a multimeter reading of the 12v from the PSU? This should show if the PSU's 12v is fluctuating. Alternatively you could swap in another PSU and see if the problem returns.

I would also check the capacitors on the mobo to see if any are bulging or leaking - could be one cause of this.

Are those readings from a multimeter? What happens if you just unplug the AC instead of the mobo connector? Does that make the +12V normal again? If not, then maybe the big power connector is loose or dirty, and prying it tight with a screwdriver wedged between the plastic and metal (both sides, left & right when connector is horizontal) will fix it. If not, then my guess is that the +12V is inadequate (if this mobo uses the square 4-pin +12V connector) or the connector on the mobo has cracked solder.

I can't figure out which becomes the problem.
The +12v can sometimes be stable at 12.01 - 12.10 for about 30-90 minutes, but then all of a sudden it starts to fluctuate ...11.80, 11.70, 11.80, 11.50, 11.45, 11.70, 11.40, 11.20, etc.
It steadily drops in 5-10 minutes until the computer crash.

This happens even when the CPU is idle. There is no pattern at all. Sometimes it happens just in 2-3 minutes after power on, but sometimes after 1-2 hours.

I also notice unplugging cable from the back of PSU can also fix the problem. The electricity in my home is stable, and I even use a 1000w stabilizer just for safety.