How do I file a grievance for bad customer service and billing issues?

Back in june I order cable service from comcast, promotion package I got was ; 3 premium channls ( 2 of the 3 were free for 12 months and 1 was free for 6 months) 2 DVR boxes free for 12 months, free hook up and installation. Comcast came out July 1st and turned my service on. Aug. comes and I have My first bill 56.00, a few days later I get another bill
saying I owe 265.00 (what happen?) I call comcast to find out what is with this bill, they say that because the sale rep. who placed my comcast order did not put the codes in for the promotion he was giving me at that time correctly (did not go through), I was being billed the full amt. because by the time the my bill came out the promotion had ended, and there was some charges that he for got to add to my bill too (like the basic cable amt).
So the rep that told me all this said that she was sorry for all this and that she could help me try to find another promotion and try to credit what she could on my account ( about 56.00) and she said that she would have to put in a request for a investigation in order to credit me more on the account, she also said to wait until I heard from them and they would let me know what I would have to pay. I waited for about 10 days then I called comcast back again and still I was told by another rep to do nothing until comcast called.
Still no call, by the time the end of Aug. comes I call comcast again, this time I ask to speak to a supervisor and was told to leave a message on there voice mail because the supervisor is not available at the time I called ( now every time I call comcast it is the same I can't talk to the supervisor all I can do is leave a voice message and they will call you back, but they never called me back). My bill is still going up and no one has credit my account and no call yet. By the end of the first week of Sept. comcast cuts my services off so I call again, this time I get a little luck and was told that they were going to credit my account 111.00 because by now my account bals was 281.00, and so the rep said that by Oct. all I would have to pay would be 169.00 and my account would be cleared up and she even cut my service back on. So I am thinking everything is find I know what I am going to pay and here comes another blow, they cut my service off again they don't have the credit on there (111.00) and I am right back where I started with this.