Worst Customer Service and Record Keeping

I can't stand Santander - they are the rudest people I have ever had to deal with. I have made payments to the original loan owner - HSBC, but they do not show up on their records. I have bank records showing evidence. I have faxed all the necessary paperwork to them, but they still don't get it right. I wish I knew what to do. They just won't get it right and now they have "added" whatever payment they say I missed, to the end of my term. I don't get it. I wish there were something that could be done about them and the horrible record keeping and customer service.