Optiplex 5070 micro 2,1 amber light (self recovered)

Optiplex 5070 micro form factor used for 4 years.

Since several days ago, when I switched on the computer, it gave amber light of 2 short flashes, a short pause and then one more short amber flash. This pattern repeated after a longer pause. No display on HDMI output.

When I unplugged the power adapter (original one, 19.5v 4.62A came with the computer) from the chasis and then reconnected it. The computer can usually(not always) be turned on and booted normally.

If I powered off the computer and then switched it on again, the problem came back. If I just restarted instead of powering off, the computer booted normally.

According to Dell's document about power LED flash pattern, the problem should be related to the motherboard. However, the computer worked without any problem once it was booted up.

Then these 2 days, the problem disappeared. The computer can be powered on normally. I have done the Pre-Boot System Performance Check, everything reported normal.

I am still wondering if the problem is related to the motherboard, or just the problem of the power adapter. The problem was so inconsistant.

I hope someone can give me some clues.