At times it charges at 1% an hour, takes many many hours to get any decent amount of charge. At times, it says its being charged via USB when its plugged in the wall charger.
Are you using the original (Asus) charger and cable?
Have you tried charging with the device turned completely off?
It's quite possible that the micro-USB port is starting to flake out. You could try cleaning it out with a toothbrush and compressed air; if that doesn't make any difference then perhaps it's time to try wireless charging.