The way to find the problem will likely be through observation.
With the opener disconnected, try opening the door manually. It should open reasonably smoothly. If there is a bind, that could be causing the problem.
Also, watch the chain when the door is being opened/closed with the opener. Is it dragging, hitting something, or jumping around? Next, inspect the sprocket. Is it missing teeth or bent?

Thanks for the reply. We called a professional and had him look at it. He found that the problem was the opener bracker had been attached too low on the door causing binding.

thanks again.