Did you remove the door from the opener (by pulling the release cord) and try the door manually to feel if it's binding? Have you checked the springs to make sure they are not broken? For the torsion springs, you often don't know one is broken, and when the second breaks, the door acts wonky.

My door was doing something like this a few weeks ago, and when I did it manually, I could see, hear, and feel the door was rubbing on one of the rails. Basically there is some play side to side in the door, and it had gone all the way to one side.