I have been suffering from this issue very often. I tried changing steps and failsafe. I printed after cubes in order to calibrate the printer. But eventually the printer decides that it needs dos stretch. I do not know what to do any longer? Any suggestion? Change electronics? Change config? Sell the printer? Dismount the printer keep some parts throw others? I just lost a huge print because it had decided that circles had to turn in ellipse .. and I mean ellipse (1,8cm x 2.5 cm) of huge error (30%) UPDATE: I have been moving the z axis up and down and measured and I observe an error below 2% when moving. The GCODE in S3D are fine, what is going on?