Billed wrong amount for 4 months in a row

Please can someone look into what is going on with the billing on my policy. In December and January I was overcharged by about R500 for each month. I had to phone and follow up and struggle to get my money back. Now last month and this month I have been billed too little and the account is in arrears. I still have to be the one to raise this and get it rectified otherwise if I had to make a claim I can see Discovery declining it because the account is in arrears. Please can someone look into this properly and investigate why I am being billed the wrong amount every month and make sure the underlying problem is fixed once and for all. I am getting tired of having to phone every month and follow up over and over to get it rectified.