I've been assembling and calibrating a Prusa i3xl from DIY Tech Shop. I'm at the point where I can print a few small calibration objects [10x10x3mm plate], but then the print flow starts stuttering. I'm printing with 1.75mm PLA at 190C. After disassembling the hot end, I find that the PTFE tubing has expanded, and there is a solid bulge of PLA inside. I tracked down replacement tubing for it, but it has just happened again after a handful of plates. The attached picture shows what has happened; I can't easily extract the tubing from the brass. Do you have any suggestions to prevent this from happening again?