Some of the horizontal lines on the box look like your Z isn't always stepping correctly. It's pretty close though, just seems to happen occasionally.
I'd turn the Z stepper trimpot up 1/8th of a turn and try it again to see if the issue is resolved.
If after turning it up 3 times (so 3/8th of a turn from where you were) this isn't resolved, I'd look for mechanical issues such as binding on your threaded rods.
For that, I'd make sure that there's no burrs on the threaded rods (or even ont he smooth rods). I usually clean up the thread of the threaded rod with some 800-1200 grit sandpaper - just wrap it around the threaded rod, pinch it with your fingers and turn - it will naturally screw through the sandpaper. I only do this once just to remove any artefacts from the manufacturing process.
I would also recommend cleaning the threads (eg: using a toothbrush to remove any particles in the threads), then use something like lithium grease on the rods to keep them from sticking. Avoid standard oil or WD40, as this can apparently weaken ABS plastic if it gets on it. I tend to put a blob of lithium grease on the threads right next to the Z axis nut, and then drive the Z axis in that direction for as far as it goes. I then put it on the other side and do the same thing in reverse. This spreads the lithium grease along the threads reasonably well. After doing this a few times remove any excess blobs of grease from around the nuts.
Note: Always lubricate your threaded rods AFTER cleaning them, otherwise the grease holds all the junk on the rod, which could make matters worse.
I'd turn the Z stepper trimpot up 1/8th of a turn and try it again to see if the issue is resolved.
If after turning it up 3 times (so 3/8th of a turn from where you were) this isn't resolved, I'd look for mechanical issues such as binding on your threaded rods.
For that, I'd make sure that there's no burrs on the threaded rods (or even ont he smooth rods). I usually clean up the thread of the threaded rod with some 800-1200 grit sandpaper - just wrap it around the threaded rod, pinch it with your fingers and turn - it will naturally screw through the sandpaper. I only do this once just to remove any artefacts from the manufacturing process.
I would also recommend cleaning the threads (eg: using a toothbrush to remove any particles in the threads), then use something like lithium grease on the rods to keep them from sticking. Avoid standard oil or WD40, as this can apparently weaken ABS plastic if it gets on it. I tend to put a blob of lithium grease on the threads right next to the Z axis nut, and then drive the Z axis in that direction for as far as it goes. I then put it on the other side and do the same thing in reverse. This spreads the lithium grease along the threads reasonably well. After doing this a few times remove any excess blobs of grease from around the nuts.
Note: Always lubricate your threaded rods AFTER cleaning them, otherwise the grease holds all the junk on the rod, which could make matters worse.