Hunk a Junk , clean and lube up your Z shaft. Your Z shaft connection with the plate, may be loose. Hold down your printer and move your plate up an down to see if you can hear a jiggle and see if there is backlash. I'm uncertain it can be adjusted, since I've never done it myself.
Worth a shot. Any advice on what products to use to clean and lube?
Also, anyone know a program to import .stl files and edit them? Specifically, I'm trying to print this large Icarus model and is way bigger than the build plate. I'd like to chop it up into sections so that I can better support it.