The 5V regulator has probably died. Non-fake replacements are hard to come by, but you can replace it with a PSU5. It's a drop-in replacement, but requires a couple of small changes to the board which are described in Pinwiki.

https://www.ezsbc.com/index.php/psu5.html#.XumsGUVKhPY

Also, the header pins do not respond well to being burnished or sanded. Once the plating is gone, the connections will be unreliable. Replace the header and repin the connector. You'll probably be doing a lot of this in the head.