Unknown issue affecting print quality
A very helpful page for troubleshooting common errors is: Print Quality Troubleshooting Guide - Lines on the Side of Print

It seems like your problem is inconsistent extrusion or temperature variation. From the photo you posted I guess that you use a big diameter nozzle. Keep in mind that your extruder might not be well equipped to deliver such a large amount of plastic consistently. This most likely is a problem with the heating capacity.

You can try to lower the speed even more to give your extruder more time to heat the plastic.