Garage door force adjustment problem

I have a Whisper Drive Model 248754 garage door opener. It has worked properly for the almost 4 years that I have lived in this house. In recent months, I have been having problems with the door lowering and then opening again. There is no flashing of sensor lights or indications of sensor problems. The manual says it is normal to have to readjust the force periodically. This is done by pressing a "Learn" button, which begins to flash. As you open and close the door, the mechanism "relearns" the force needed for proper operation--at least, in theory. When it has learned, the light stops flashing. You have to go through a full up/down cycle to complete the learning process. I have gone through this several times. Sometimes the door operates properly. Sometimes it takes 3 or 4 tries to close the door and keep it closed. The manual does not indicate any manual adjustment option (e.g., turn a screw). Any suggestions as to how I might solve this?

 

Thread moved to proper forum. Duplicate post removed.

 

How does you door run by hand? Does the belt appear to be a little loose? Unplug the opener, remove the circuit board and check the gear inside is there black powder on top of the white planetary gear?