Bad customer service

I have had my credit card since 2010 from Absa. All of a sudden since April 2015 Absa started charging me an interest of not less than R250 every month. this interest is still being charged every month to date. I have made several call to Absa and to the credit card division. When ever I call to ask what changed in April 2015 to cause the interest to be charged all of a sudden. they say to me that they can not go back so far on their system and that they will get another agent to look into it and call me back. I wait and nobody from Absa calls back. this has been happening since Sept last year that I have been trying to sort out this problem. the latest call I made to them was on the 7th of Jan 2016 and yet again they said to me that they cannot go back so far on the system and someone will call me back. 7 days later and again no call back. at this point my instalment on my statement Is R330 and the interest is R253! So after they take off the interest R80 gets paid to my credit card of which they then take of R37 for bank fees!!!! this has been happening since April 2015! I have no idea what is going on because I am paying interest only!!!!