Gate repair help - start/stop points won't stay in place.

I'm trying to get our gate to open the proper position and close to the proper position. It seems to be a problem with the start/stop points. Here is a photo of the stop point when the gate is open: http://imgur.com/RAN8VQP

I push back the plate and can manually adjust the white rings to change the open/close points. When I do this, the gate works properly usually for a day or so. Then, the positions seem to change and the gate tries to open too far, hitting the stop point and opening back up. As a result, I cannot close the gate until I manually readjust the start/stop points again. When this happens, the "open" point of the gate is clearly wrong as well as the gate stops a little shorter than usual when opening.
It seems fine when I watch it closely, but then once I close everything back up and run it a few times, it pops out of position again.

Here is a video of it in action: https://vid.me/SiLG

Any thoughts on why this keeps happening??

Thanks!