When the print is at the center of the build plate , there is some deflection so that the table moves slightly up and down, affecting the quality of the print. As the table is only supported from the back the only thing i can imagine is to keep models as close as possible the the back and slow down....I forget what speed this was happening at... Ill keep better notes in the future , now I am printing at 50... and infill at 70-100 or so,