Random overextrusion?

Hello, So I have a Creality CR-M4 (ratrig on its way) and I've been getting really bad uneven layers on some parts of prints with PLA. And some parts are looking really good. I checked the following things: -PID tuned bed and hotend -calibrated e-stepps -printed with lower print speeds (current at 55 mm/s) -tensioned bolts, belts, ...... -printed with other temps Anyone happends to know what the problem could be? Thanx in advance!