I do have belt tensioners on all the long belts and they seem reasonably tight. Also, the jump only happens once during a print (even the 1500 layer print that I did last night), and it always happens on the same layer (2nd layer not including raft). It seems that it is related to either software, firmware, or electronics. I just put an Ultimaker controller on, and I think that it may have something to do with that. I installed the version of Marlin that comes with Cura 4. Is that compatible with the Ultimaker controller?