Since the representative assigned to you is no help, I would recommend going to his/her superior. You can either say something to him/her like "Listen, you don't seem to be able to get this straightened out and my Dr is getting extremely upset so I think I need to talk to your manager." If you're not comfortable with asking them directly for their manager then you can call in on the main number and ask for the name and number of his/her manager and just call directly.
In either case, usually when you push to go up a step you will find action gets taken. Either the original representative will suddenly get things cleared up, or their manager steps in and fixed it.
You can get very firm without getting rude. They are messing with the Dr's income so I never fail to mention how upset the dr is getting. You may want to consider mentioning that the dr is considering dropping out of the network. He can't afford to not be paid.
Sometimes we are too patient and when we put our foot down (Enough is enough!) things get cleared up much quicker.
Good luck!
Michele