Citibank — interest charges being debited in my credit card statement since past 7 months

I have been using citibank premier miles credit card since 2016. Despite a timely payment on a regular basis, time and again from august 2017 i am being charged interest + gst. Have tried contacting and talking to the bank via emails, letters and phone calls but no solution has been given yet. 2 days back, got my statement for february and interest charges as a fixed mandatory charge is again reflecting in the statement. Was told by the helpline executives that the bills are system generated so will check and rectify it, but after almost 6 months they are still unable to do so. This shows how this bank force 11 year old customers to stop any communication with citibank. After numerous attempts and escalating my problem, there is no logical and reasonable explanation from the banks end regarding the baseless and false charges.