Your problem is most likely caused by a small printed circuit board just in front of the motor. It is the RPM sensor and the part number is 41C4398A. I would really be concerned about the down force being set at maximum. This affects the safety of the opener. This and it being a pre-door sensor model make a good case for replacement.

Don,
I have the same problem with a Craftsman garage door opener that one of your other members had. I replaced the large nylon drive gear and after reinstalling the motor & chain per Sears instructions, the door moves up about 1ft and then reverses down about 1ft and stops. The black interrupter cap is intact on the end of the motor shaft. I have not looked yet for the RPM sensor, but after finding it, how will I know if it is defective? Any other ideas? This seems to be a common problem. Thanks

Make sure you put the black cup back on the end of the motor shaft and that it is pressed all the way on. Make sure the RPM sensor is plugged in. If neither of these are the issue, then you probably did not have the motor shaft pushed all the way back when you tightened the coupler after replacing the worm gear. Loosen that coupler up and from the front of the motor (opposite end in which the coupler is on) push the shaft back as far as it will go. Hold it there while you tighten the coupler back up and then try it.