When you pull on the thread, do you have the pressure foot up? It took me a while to realize that having the foot down engages the tension!
You can try running some dental floss (I think the unwaxed kind) through the tension mechanism.
Have you cleaned your bobbin area recently? Sounds like the thread you are using might be very "linty".
Hope this helps. I think the 6500 is this generations "featherweight" in that they will be around 100 years too!