Most of this is just off the top of my head, not necessarily the best legal advice your going to get. But I can tell you to ditch the class action idea. It's not likely to be in your best interest, and probably wouldn't be all that viable.
Per an individual lawsuit, and again this is just a quick, initial thought: you may want to consider an action for breach of contract. If you don't mind giving up the car, file a suit claiming that they breached the implied covenant of good faith and fair dealing by constantly harassing and threatening repossession when the account is not past due.
Ask the court to rescind the contract and order a return of all money paid in exchange for your return of the vehicle.
There may be a better option out there, but this could be worth a shot. Maybe the threat of such a suit will get them to behave in a more professional manner.
Best of luck!