ok, it lifts and lowers ok right?
only when it transitions from full free lift to start staging and the channels start telescoping in that first part of the channels starting to extend you get jerking?
i would pay attention to the rollers, look at how they track, when extended up past that point look in the channel tracks for any signs of rubbing. There would be witness marks if any unusual friction is taking place. If there are any thrust plates or wear strips they might be tight causing it to be tighter than usual and make it jump like that. If you want you can apply some grease to the channels and see if that improves it. It's not too hard to do, just get a painters tray and a roller with a long extension and just roll the grease on. Run it up and down a few times and see if the jumping stops.
Note: normally you wouldn't need to grease the channels and i don't recommend that because grease attracts dirt which in turn causes more wear so cleaning the grease out and applying new grease would have to be part of a pm routine if you started doing that.
With that being said i would pay attention to how tight those rollers are, how they are shimmed. Also measure the distance between the bottom rails of the channel compared to the top rails and see how close the rollers are on side thrust compared to the bottom. Like i said, if the top is narrower then the rollers might be binding from tight tolerances causing it to hang up and jump like that.
If you are not comfortable doing this then get a professional forklift tech to look at it.
But you can at least try the grease thing for now to see if that changes anything.