Cracked it - my N4M was failing everything because of one loose belt

I had a similar mess with my printer a few months ago but the issue wasn't the belts; it was the Z-axis lead screw nut. I was printing a larger enclosure, maybe 200 square centimeters, and the print would start fine but then the nozzle would just start bouncing vertically. It wasn't doing a full line of junk like your situation, but the consistency was completely shot.

I figured since the frame was solid, the tolerance in the nut must be the problem. I checked the nut on the Elegoo frame and found that it was a tiny bit loose on its housing. After a week of frustration, I carefully unscrewed the nut, inspected the bearing, and tightened it just a hair with a small hex wrench. It took about three minutes total. Once secured, the vertical jitter stopped imediately. It felt like such a minor adjustment, and it was much less of a pain than spending hours chasing a print that looked perfect for the first layer and then just collapsed into a pile of molten plastic.

My expirience with movement issues turned out to be less about internal hardware failing and more about the entire setup. I had this Elegoo setup sitting on my workbench, and even after ensuring the belt tension was perfect, the print quality still looked terrible after about an inch or two of travel. The lines were all jagged and inconsistent, even when running simple squares. I spent most of my time checking the printer itself, thinking it was a belt or perhaps the stepper driver timing, which is what the previous poster suggested with thier findings. However, after moving the unit onto a heavy, isolated mat and checking the level of the entire frame with a bubble level, the quality instantly improved. It wasn't the print head shifting, but the whole machine vibrating as it moved across the surface. It was a subtle thing, but having enough stability beneath the base made a huge difference in how cleanly the extruded material came out. If you are fighting wierd inconsistencies that don't seem to relate to a loose component, maybe try isolating the footprint first before assuming there is a major mechanical failure in the printer itself.

I once encountered a similar intermittent movement issue where the printer seemed to travel just fine but the printed geometry exhibited subtle stair-stepping even on very small features. After ruling out belt slack and lead screw play, I determined the root cause was excessive play in the guide rails themselves, specifically where the linear carriage mounts interface with the upright structure. It requires a fair amount of careful inspection to see if the carriage is simply vibrating against the guide rather than gliding smoothly along it.

It appears that sometimes the sheer rigidity of the frame isn't enough; the mechanical interface needs precise tolerances to prevent that micro-shift during extrusion. It was a surprisingly fiddly fix to tighten those mounting screws just enough to kill the lateral wiggle without introducing friction that would negatively affect the print speed.

I had something like that happen too, but it wasn't the frame. On my N4M, the issue was the stepper motor wiring. I wasn'd get this intermittent skipping wich looked just like the carriage shifting. Turns out the jumper at the controller's end was a little loose. I just checked the connections and gave them a proper twist with a small wrench. It fixed the jitter immediately, so I'm always checkin' those signal cables after a major move or if the printer seems off. It's crazy how small a part can mess up the whole thing.

I ran into this exact movement instability once, but it wasn't the movement hardware itself. It was acutally the main assembly bolts securing the frame to the surface. During my last run, I realized one of the mounting screws at the base was only finger-tight and had loosened by about a half rotation. Once I tightened that connection, the rattling and perceived wobble vanished immediately. Check those foundational bolts if the belt and axis adjustments are alredy solid.