Usually i would think it is something to do with Z binding but Looking at the string test image it doesn't look like its a Z issue as each tower has bulges at different layers.
I recommend looking to see if the bed or the x gantry has any wobble, also look to see if the extruder is gripping the filament properly and not grinding it as this was a problem for me at one point. Also just in case it is z axis related look to see if the bulges are on all sides of the xyz cube on a single layer or if the layer has shifted because if its even all round the object then it is probably z axis related. An easy test is to manually move the z axis to see if it is hard to move or not and check to see if the rods look bent at all. to me it looks like a mechanical issue but try tweaking the z seam to be all in one line if not already and look to make sure your filament diameter and nozzle diameter settings haven't changed.
If your still having problems after more tests uploading your fff profile would be helpful for people to input on your settings that could help to fix it.