The Autobike was indeed a self-shifting bike with weights on the rear spokes that moved outward under centrifugal force to operate the shift mechanism. It was sold via "infomercials" and is generally reguarded as a very poor quality bike. The OP's 18 years of service from one is astounding. Most died very quickly.
I agree that getting it repaired is going to be difficult if a simple infusion of oil into the freewheeling mechanism doesn't solve the problem.