Lubricant should not ever be used on Kessler sliders, it simply will attract dirt and in the end create more issues.
There are a number of things that can effect the smoothness of a slide.
There are a few things that you can check to help determine where the issue is coming from.
The first thing to check is to remove the drag knob and disc.
Does this fix the problem? If it does are there scratches/gouges in the shiny metal surface of the drag disc?
Also, with these 2 items removed, watch the drag shaft while moving the cart quickly to see if it is wobbling or not straight.
Locate the black washer and black o-ring and make sure they are properly inserted into the drag cap (black o-ring in first, then black washer)
Remove cart from belt:, this will determine if the issue is part of the belt system or the cart itself:
If cart still pulses with belt detached, then a cart adjustment may be needed:
If you have performed all of these steps and were unable to fix the issue your slider will likely need to come in for repair. Please reach out to us at firstname.lastname@example.org and let us know what you have found and include a video showing the slider so that we can better troubleshoot.