Garage door jerks when closing

New to the forum. Happy Memorial Day everyone! 

Here's my puzzle. My garage door tension spring broke a couple of days ago. Cables scrambled, motor probably jerked a couple of times back and forth with very loud noise. I watched youtube and installed new springs and it looks balanced to me (stays if pulled up 1 ft from the ground). And manually I pulled it up and down a couple times, greased, everything seems smooth. Old squeaky noise is gone too. 

Now once I hook up the opener (liftmaster 8355, belt drive), it jerks back and forth when closing (especially the first 5 secs), less so on the rest of the trip down and much less noticeable when pulling up. When I climbed up, I can see the spring that connects the belt squeezes and releases when the door jerks. I am pretty sure there is nothing in the track. 

I thought it could be the gear/sprocket worn out. Opened the cover only to find it looks clean inside. Took a couple of pictures and a video (when door closing) showing how it sounds (maybe the motor?). Really not sure where I should look next. Maybe the trolley is worn out? 

Here's the video. 




Please help and thank you all in advance. I thought I avoided a bill from changing the spring but seems there is more headache.

If your garage door opener (LiftMaster 8355 belt drive) is shaking or oscillating when closing, especially in the first 5 seconds, it could indicate an issue with the door's balance or the opener's force settings. Here are some steps you can take to address the problem:
Check the balance of the garage door: Disconnect the opener and manually operate the door. It should move smoothly and stay in place when partially open. If the door is difficult to move or doesn't stay in position, the springs may need adjustment or replacement. It is recommended to have a professional technician inspect and adjust the springs if necessary.
Adjust the force settings on the opener: Most garage door openers have force adjustment settings that control how much force the opener uses to open and close the door. Consult the opener's manual for instructions on how to adjust the force settings. Gradually increase the closing force until the door closes smoothly without shaking, but be cautious not to set it too high as it can cause damage or create safety hazards.
Inspect the belt and opener components: Check for any loose or damaged components, such as brackets, pulleys, or the belt itself. Tighten or replace any parts as needed.
If possible, we are happy to provide online guidance and help, you can contact us: CHI Hardware Co., Ltd.
Or you can directly send us an email about your problem, and we will reply to you as soon as possible after seeing it.
Email: chi@chihw.com