Wells Fargo Dealer Services Untrustworthy Thieves Santa Ana, California

I have been a customer since I purchased my Mercedes Benz in September 2010. My payment is around $500/month. I pay $300 every 2 weeks automatically. 

Last year I looked at my account online and saw they had increased my payments even though I had paid 4 months ahead. They couldn't explain it and eventually after alot of tense moments and time stating my case, they reversed it. 

I go into my account about a week ago and notice they have again increased my payment. This time they threaten to report me to credit bureau. I asked the representative what she was going to report since my August payment is actually paying off January 2013. She couldn't answer my question. 

The supervisor got on the line and could not answer my questions as to the logic of increasing my payments when I am 4 months ahead. She referred me to the regioanl office. They increased my payment by almost 50 dollars for January 2013.  Who do I report this lunacy to and how can I resolve this? It's so upsetting!