Can someone help me please?

i purchased a car with GMAC, in the year of 2002 . with 72 months to pay it off, abd my payments are due on the 29th of each month, i send it in on time everytime, but they are saying I'm always late, and keep adding late fees, on top of that their telling me i missed 2 payment, but on the other hand the all the checks are beeing cashed,and taking out of my account, I've tried to resolve the problem, with managers of GMAC, and the Better Bussiness Bureau, even with the Office of Consumer affairs, nothing helps... their saying i still owe them $1.20.30. i know I'm done paying for this car, and has all my papers, and cashed checks, what else can i do? if i don't keep sending them money, they will come take the car away. helpfuf Answers only please.....