Few things,
Could be filament related, if you use filament that has an inconsistent diameter then you will get wider layers where the filament is fatter. To check this unroll a meter or two of filament and check the diameter every say 100mm. then take the average of those readings and put it in your slicer as the filament diameter.
Could be temperature related, If your hotend is not tuned correctly it might overshoot the temperature when trying to keep heat. when the hotend is hotter the material can flow better and more filament is extrude. Run a PID tune for you hotend to make sure you get a stable temperature.
Could also be mechanical related, If your lead screws are not parallel and straight you will get z wobble, you can tell because the lines on the side of the print will always be in the same position. Could also be vibration in the y or x axis, tighten belts and try to reduce moving mass. With a 3D printer you want a nice strong square (as in true) frame. Sadly most of the printers on the market do not come with this and require upgrades.