If you have a Chamberlain (or I think Sears Craftsman) model it potentially has a sacrificial plastic worm gear in it. That can get worn so thin it barely drives the door. In my case the door stopped opening at certain points as the teeth simply folded over instead of transmit the torque.
If you were close to that state of wear it might be what is actually happening is that the worm gearset is failing to "lock" and instead the door is "overdriving" the weak teeth.
Along the same lines its possible something is slipping in the drivetrain between the chain-driven carriage and the electric motor.
The good news is if you had that fail mode, parts are available all over, Amazon, etc, and around $25.
This is exactly what happened to one of the Sears chain drive garage door openers at my dads house, and reason I suggested the process to the OP to see what was going on with his door opener.
My dad purchased two Sears Chain drive 1/2HP garage door openers back in 1988 for his two car garage. He used one door much more often than the other door, and as a result, the most often used door ended up having issues after about 15 years, similar to what the OP describes.
It ended up being the white plastic worm gear inside the motor drive housing. My dad wrote to Sears about it, and believe it or not, they sent him a brand new worm gear for free, even after all that time had gone by. I was amazed.
The original worm gear was worn in such a way that would let is still work, but barely, and make a lot of noise and a "jumping" motion as the door would go up and down. It was also fairly easy to push the door open after it went down. It was these reasons that my dad took the door opener down and check to see if he could figure out what the issue was, which he did.
We took the motor drive assembly apart, replaced the worm gear, and the door opener worked great again. As a matter of fact, we sold my dads house in 2018, and both of those original door openers were still in the garage and still working.
Jim
