Hi. I have seen this before, although usually not from chamberlain garage door openers.

Pull the release cord that unlocks the door from the opener, and manually open the door ALL the way open.

Then lock back in place in the open position.

Then use automatic opener to shut door.

It will close very slowly, but should close all the way.

Then, open the door again using the remote, and the door will once again open slowly, this time it should open all the way.

Then close door with remote, and door should now close as it is supposed, with the proper speed.

The slow opening and shutting is the system position sensor resetting itself.

This is why your door is acting up.

I would also look up by the belt drive connects to the front of the garage door opening, to make sure there isn't any dirt, cobwebs, or other obstructions on the sensor that may cause it to re-close the way it does.