It's probably filling like that because your drain hose is not 38"' from the floor in a standpipe as it should be, or you have it sealed off somehow. If the water siphons away before it can fill the tub, you'll continue to have problems. More on this here
If you have the drain jerry-rigged because it backs up, then prepare for pump and other damage later on. In fact, you should stock up on those drive couplings that AJ has posted on the right side of this page. You'll need 'em
The overfilling is most likely a bad water valve....a common enough failure which can be intermittent. No doubt the valve has been overworked in your case due to siphoning and refilling constantly. Very inexpensive fix all the way around.