Product: Credit card or prepaid card

I signed up for a zero percent balance transfer from XXXX XXXX XXXX XXXX in XX/XX/XXXX. I had a balance of XXXX of purchases on my account at the time which were due by XX/XX/XXXX. I made a payment of XXXX on XX/XX/XXXX to cover these purchases along with the balance transfer fee of XXXX, so I would only have the XXXX amount on the zero percent to make payments on. I noticed on my XX/XX/XXXX statement that I had been charged interest of XXXX, I checked back on my XX/XX/XXXX statement & I had also been charged interest of XXXX. While looking through the statement I noticed under promotional rate they only listed XXXX & had put XXXX under purchases when in fact all purchase amounts had already been paid on XX/XX/XXXX. I called customer service & spoke with a rep who said they would take care of the interest & fix the problem. When I received my next statement, I had more interest, XXXX and no credit.XX/XX/XXXX, I called back into the service center & another rep, XXXX, told me they would issue the credit but they had to send a request to another dept to take care of the problem. 
They issued credit for the finance charges, but they also removed my initial payment of XXXX ( which I did not request ) but that did take the amount out of purchases so all I had was the amount under promo rate. I called in XX/XX/XXXX, nothing was resolved yet. On my XX/XX/XXXX statement, they added the XXXX back to my account but they put it under purchases, not promotional rate. On my XX/XX/XXXX statement, I had more interest XXXX. I called back in to tell them they are still charging me interest & now they have put the XXXX back under purchases after they removed the XXXX that they had under purchases originally, so we are back where we started. I asked to speak with a supervisor and was told there was not one available. I told them I needed to speak to someone who could fix this issue and I was put on hold. After 30 minutes, I hung up. 
My next statement, XX/XX/XXXX I was charged XXXX interest, XX/XX/XXXX I was charged XXXX, I called back into the customer service center XX/XX/XXXX, told the rep my entire story again, she said that she would need to send a dispute form to get this all taken care of, she said it would be taken care of within 60 days. 
In XX/XX/XXXX I received a letter from the dispute dept stating they were correct with how they charged this and then on my statement they put XXXX back on my account under purchases and then charged my more finance charges of XXXX. 
I called into the service center and said I really needed to speak to a supervisor & I was finally transferred to someone who said they were a supervisor. He ( XXXX? ) said that a supervisor could have fixed this problem from the beginning and that the reason I had this problem is that they had changed the way the computed the interest about the time I got the promo rate, he apologized & said it should n't have affected my account this way and said he would have it all fixed for me & get the interest credited. He said to wait a couple of weeks & then just call back to verify the total amount of my account was all under the revolving account. I felt like this time it would be taken care of, finally. 
Then, believe it or not, I get my XX/XX/XXXX statement and I have finance charges of XXXX on the statement and then I see the total amount under purchases at this point and it is XXXX-I am being charged interest on this crazy amount when I have made absolutely no PURCHASES. Another call into the customer service center, I got a rep who wanted to know the problem, saved her the trouble of looking at all the notes, gave her the entire story, told her all she had to do was issue credit because at this point I had no confidence she could fix the problem, just make sure the interest was credited. I just did n't have the spirit left, I had to get ready for XXXX. 
XX/XX/XXXX, I am ready to proceed to solve the problems again. I call the service center yet again. I decide to start documenting the time I spend on hold because everytime I call in I am on hold for very long periods of time & if I am transferred, I am on hold even longer, or better yet, disconnected, so I have to call back and start all over. This happens at least 50 % of the time. When I initially call in I am on hold for 20 minutes before reaching a rep ( XXXX ) I explain the situation and state that I need to speak to a supervisor. XXXX ( I hope this is a real name, I am also starting to write names ) transferred me to a supervisor, XXXX ( I was on hold 14 minutes ) I explained the situation to XXXX and before XXXX could tell me what he planned to do to correct this problem, I was disconnected. Not really sure how this happened, but it has happened to me before. I called back but after holding 15 minutes, I gave up & decided I needed to just move this issue up to someplace where it may get some attention. Help! Help! Just as a side note, my zero interest promo ends in 2 months. I do n't really feel like I have had that yet. 
I have documentation if needed, on all of the information I have provided.