X-axis layer shift, possibly bearing issue?  

X-axis layer shift, possibly bearing issue?

I've been trying to troubleshoot an x-axis layer shift that now may be due to the bearings.

Starting with the layer shift, it seems repeatable but not across all my prints. It seems to depend on the general movement. Smaller prints near the middle are fine. I noticed it almost 100% when trying to print some lithophanes that are just basically long slow left and right movements. I've had it happen on the odd other print, but very rarely. With my current project, I get a 0.7 or 1.3mm layer shift once or twice a print.

Some photos of the attempts/failures. https://imgur.com/a/TFgWceD

I thought at first the belt might be loose and was riding up on the idler's edge, causing it to effectively change the tension but even when I caught it in action, the belt wasn't riding up. I gave the idler bearing a light lube anyway. Belt tension reading 275 and makes the base tone when I pluck it, so that seems ok.

Then I wiped off the x-axis rods, lots of black deposit I was assuming was fibres from the belt. I found the x-carriage easy to move by hand, but would hitch somewhere around the middle, not always at the same place and not always from the same direction. I was also sometimes getting a "crash detected" error when I was watching the printer move around the middle and had nothing it was contacting, so I'm suspecting it is the bearings causing all my problems.

So, I oiled both rods and no longer hitching. Great, however now I have a new symptom that wasn't there previously. I now get a "clicking" when the carriage makes fast movements from right to left, around the middle of the rod. it sounds like the ball bearings inside the linear bearing are clacking around and collecting when it moves in one direction. You can hear it at 0:03, 1:03 and 1:10: I checked the rods for pitting/uneven wear but couldn't notice anything. Also, even after having just wiped down the rods, touching them already leaves a black residue (like graphite) on my fingers. I'm currently running my first test print since all the maintenance.

So what I'm trying to figure out is:
1. Would just the bearing hitching like this cause the layer shift or should I be looking into something else?
2. Will it hurt anything to continue running with the bearings clattering like this?