If you don't get any help here, go to The Garage Door Man. If you don't see any help for your specific question, you can e-mail the 'man'.
When you say jerk, just to be clear, does it jerk up, then go down normally? Or is the jerk more like a slight drop, or free-fall, after which it goes down normally? Just a clarification though I can't help you. If you end up posting your question at the man's site, perhaps there you can clarify.
Happy trails and hope this helps.
PS: If you're wondering how I found this site, I went to my favorite WEB search engine and entered in +"garage door opener" & +problem and pressed Search. This VERY general WEB search generated over 4100 'hits'!!! (As an aside, familiarizing yourself with a search engine is a great asset if you're not already familiar with one.) Good luck!