If you're experiencing problems on the Z-axis, there are a couple of possible causes, depending on the exact issues you're seeing with your print.
If your print is exhibiting a bulge somewhere on the Z-axis and it happens repeatedly, but in different places each time, then your Z-screw or trap nut is probably dirty. Use a piece of cloth or a brush and some rubbing alcohol to clean the Z-screw. There is no need to disassemble any part of the printer for this; simply move the bed up and down to be able to reach it all.
Re-apply the green grease that came with your printer to the Z-screw, and use the Maintenance menu on your printer to move the bed up and down. Go to Maintenance --> Advanced --> Raise Bed. This will lower the bed to the Z stop and raise it back up. Do this several times to spread the grease out. If you no longer have the green grease that came with your machine, it's called Magnalube and is available online.
If the bulge persists, you may need to remove the trap nut and clean it as well. A wire brush and soapy water will usually do the trick. Make sure it's dry before putting it back on the printer.
If the bulge is staying in the same place every time, there may be an issue with your Z-screw, or your bearings. Put a couple of drops of sewing machine oil at the top of the Z-shafts on either side of the Z-screw and run the bed up and down as described above. This will lubricate the bearings as the bed rides up and down on.
Evenly Spaced Bulges
If you have regularly spaced (generally 3mm) bulges in your print, something is going wrong with the revolutions of the Z-screw. This is usually indicative of a problem with the motor in the Z-screw, but do make sure the screw is cleaned and lubricated first, before any disassembly.
Irregularities on Different Sized Parts of Same Print
The two prints shown above only have irregularities on the tops of the columns, and not on the rest of the print. This is indicative of a temperature fluctuation problem. It's more visible on the tops of the columns on this print because it's over a smaller area.
If you have a contact temperature probe, touch it to the nozzle in the menu, Maintenance -->Advanced --> Heat Nozzle, with the nozzle at 230 degrees. Hold your hand steady, and keep an eye on the read out both on the printer's screen, and on your temperature probe. It will fluctuate a bit until it settles out. Once it has settled, a fluctuation of 1-3 degrees is normal. 10 degrees or more can cause issues. If the temperature is fluctuating, a new temp sensor should resolve the issue.