I had the same issue. Suddenly my iPad wasn’t charging properly. It was restarting every 2 seconds. Brought it to the Apple Store to have it checked. Battery was ok. But they couldn’t fix it (hardware issue) and offered to buy a new unit.
I ended up buying a new unit but didn’t gave up on the old one.
I brought it to someone doing micro soldering and upon opening it he found out that some component of the charging board had to be replaced. It did the trick. Cost me 159€. ;)