DIY printer, extrusion problem, Titan/V6 clone

Hi,I'm testing my DIY printer and have run into a problem:Some layers appears to be under extruded but not all. A few under extruded layers, then some OK layers and then it repeats. I think this must an issued with the extruder/hot end, but I don't know what..I have attached two pictures of a typical Benchy printed from PLA at 210C with 0.15mm layer height.Observations/things I have tried:-There is no grinding of the filament.-I have adjusted the spring on the extruder: No change.-Tried print temperatures from 190 to 230C: No change.-I have tried 1 to 6mm retraction with speeds from 15 to 25mm/s: No change.-Disassembled the hot end and put it back together: No changeReduced part cooling: No change (other than the expected artifacts from insufficient cooling.)Printer main specs:-Clone Titan extruder, Clone V6 hotend (direct extruder)-0.6mm nozzle-CoreXY-200x200x300mm build volume-Built from 2040 V-slot profiles-Lead screw Z-axis-Heated bed-SKR1.3 board running Marlin2-TMC2208 stepper drivers-3D Touch probe