Prusa i3 Printing Issues

I recently built a Prusa i3 from a kit and noticed that all of the prints are not smooth and have these bulging layers. To fix this, I've tried tightening the belts, oiling the rods, and printing slower at 20mm/s but saw no results. I am a newbie to 3d printers and does someone know a solution to this problem?

It looks very much like z-wobble. Are your Z axis threaded rods fixed at both ends? They should be free at the top end, or any bend in the rod will translate to the effect you're seeing in your print. You should also have a flexible connector connecting the Z axis stepper motors to the threaded rods.

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