Cause of iPhone battery issues?

A few things to check first before going for more expensive options:

Check charging socket for any lint / blockage as this stops the cable from going in fully.

If it is not going in fully and has a gap you need to clean with fine straight tweezers carefully without touching the 8 pins at the bottom of the charging socket.

Try another charging cable and or wall charger, it's not rare for them to go faulty or worn out.

If you have a microscope at hand inspect charging port for any pushed down pins. As these case cause a bad connection to the charging cable, in some cases pushing the cable in the direction of the charging pins would temporarily regain connection until you stopped applying pressure to it.

It sounds like Tristar charging chip damage. Was the computer detecting the phone e.g via iTunes or was it only charging from it?

That chip can only be replaced via micro-soldering and is done by people that do board level repairs for iPhones. You may be able to find someone locally in your area that does them.