Intermittent faults can be difficult to locate. You may have to try by a process of elimination and then by testing.
By this I mean for example, replace the DC-In jack part #5C10L47427 (supplier example only - search using the part number only to find others that suit you) and check what happens. I realize that you said that it was checked and that it wasn’t the issue but you also said that the problem didn’t occur then.
Here’s the hardware maintenance manual that may help with this. Go to p.50 to view the procedure to remove/replace the DC-In cable
I’ve had intermittent faults happen with the same connector type due to the centre pin connection of the connector being broken inside the connector itself and only making intermittent contact with the wire it was connected to. Sometimes they were was good for days and then it wouldn’t work at all etc. Unless you can definitely prove that it doesn’t fail i.e. mechanically stress test it by connecting an Ohmmeter to the input and the output of the connector via a cable plug (both connections pin and barrel) and then move it and the plug as best you can to see if the connections remain solid and do not waver. Then if there is any doubt you may have to change it
If it still occurs after you’ve either proven the connector or changed it you would need to have the schematics of the motherboard and trace where the power circuit went from the connector and start testing to check if the input voltage remained steady etc
Schematics can sometimes be found by searching online for the motherboard’s board number which is found printed on the motherboard i.e. search for (insert motherboard board number) schematic. I’m thinking that this is what the computer repair guy would have or should have been doing if the DC-In jack was OK but can understand that if it didn’t fail when he was testing then it is difficult to know what to look for, beyond potentially faulty solder connections or components that don’t quite look right e.g. heat stressed or cracked perhaps. As I said, difficult.
Out of interest does the charging light turn off or does the battery stop charging? Try creating a Win 10 battery report and check the status of the battery to see that it is OK and not maybe impacting on the laptop.