I'm thinking you have a mechanical problem .. not a thermal problem. Put a one-tenth indicator on the turret ... put it against some turned stock or something rigid .... in .0001 jog mode click it ten times one way ... then the other way .... see if it returns on the indicator to zero.... Also the nut housing might be the problem .. check parameter 138 on your machine and let me know what it is ... the nut housing might not be perpendicular ... this would cause friction and heat on the X-axis ballscrew ... The machine will hold tolerance ... we just need to pinpoint the problem .. sometimes everyone is looking at the most technical thing .. when it is something really simple. This seems like the case. I will help as much as I can ... just give me a little feedback.