Discovery Bank Credit Card interest charges

I didn't pay up my credit card in full one month and Discovery charged me interest for 2 months on my next statement - they added an interest adjustment for the previous month that was paid in full. When I phoned to query was advised would be forwarded to the product dept. No response and not resolved on my next statement. I was actually charged interest for a third month running even though the previous statement was also paid in full. Phoned to query again and told it is 'memo' interest - a penalty for failing to pay previous month in full i.e. get charged interest for the next 2 months! Still no reason for the back interest charge of the month prior to not settling in full - agent now claiming it was for putting fuel on my card (that I get charged interest on all transactions for this single transaction?). I've been paying for fuel on my credit card for months without interest being charged. I cannot understand why they seem so unwilling to resolve my query and reverse the charge that was made incorrectly (or provide a VALID reason for it). I'm starting to regret my switch from FNB - was going to cancel it this month, but now having second thoughts!