Have you looked under the slides to check the darco? Is it damaged, frayed, abraded etc? If so you will need to determine how bad it is and then proceed from there.
If it has sat for long periods and not been used has the darco become dirty? The darco on the bottom of the slide "glides" (or it's supposed to) over a wear bar underneath the slide. If dirt, crud or other debris has gotten into that space it can make the slide jerk. If the bottom has any other type of substance that could make it jerk. If it has sat for a long time the seals may have become deformed and just curled up around the edges of the slide causing it to jerk. Lots of things. The seals themselves could be compromised in some way sticking to the edges of the slide causing an issue.
You can try the rollers mentioned in a previous post but the link was to the roller for a 1/2" wear bar, they also come in 3/4" so you need to know the height of the one you have if you go that route. A note on that though - I used to want them myself but I have now seen multiple slides with the darco worn through/torn up by rollers. They did it to our new MHC the first time the slides came out. Keystone replaced the bottoms of my slides with a solid material so my issues are gone but that was under warranty (good will
).
Look those slides over top to bottom, side to side and post back with anything you find. If it has been sitting that much I would clean the seals for sure then treat them. I put 303 on my seals; just rub it on, let it sit a bit and rub it off. Get back and maybe we can get it working without doing some major mod.