Unauthorized change to credit card system

I recently noticed that FNB only deducted 10% of my Petro card account from my current account for 2 months. I was then being charged 15.55% interest on the outstanding amount while getting virtually no interest on my current account which has ample funds to cover the card. After numerous emails and telephone conversations (to totally *********** people who were clueless in every aspect), I finally received an email (29 Oct) informing me that the interest would be reversed as they had loaded me onto a revolving credit system. I assumed that this would also be rectified. Well today it happened, my Petro card was declined due to insufficient funds. Embarrassing, inconvenient and downright irritating. I have numerous accounts with FNB but this obviously amounts to nothing as there seems to be a combination of incompetence and an attitude of not caring with them.