Have a Chamberlain model WD832KD belt drive opener. When the door is closing from the open position, both the trolley and door jerks back and forth a few times, then settles down smooth the rest of the way to it's closed position. Opening is smooth with no problem. Disconnecting the door from the opener & manually operating the door is perfectly smooth & properly balanced at midpoint." Nothing wrong with door".

However, when the door is in the open position, I can take and move the door back and forth and can see some play as the trolley moves back and forth on the rail. So, I took the cover off the opener to check the nylon gears. They are both good and well greased. The spring loaded connection that connects both ends of belt is adjusted according to manual.

I am almost certain that this excessive play is causing the problem. This constant jerk has to be hard on the opener! The problem now........is that I don't know what to look for to correct it.

Waiting to hear from people that know more than I do about garage door openers.