You've replaced some parts but did you take the time to properly set up the rear derailler? It sounds like a simple tension issue based on your description but given the mileage on the bike and the fact that you've fiddled with the adjustments already, I would suggest starting from scratch on the set up. If you still have a problem after that, then start looking at replacing components. www.parktool.com has a great rear derailler set up procedure. Follow it step by step.