topcon excavator 3dgps problems

We are running all Topcon, and one of our rigs is a cat 374d excavator. Has anyone heard of the Topcon system changing control files? I had it re-calibrated a couple months ago and it was in tolerance for a little bit but I noticed it would get further and further out of tolerance. after about 6-7 weeks I checked the control files and the N. and E. had been "adjusted" about a foot or two on all 6 points. The Topcon rep and I had noticed 3-4 of them had been changed when he was calibrating, and I chalked it up to operator error. Now all the points match but it is still off a foot +/- horizontal and a foot vert. Any ideas would be helpful. For the record after it was calibrated it was still off 6 feet horizontal... I corrected the control points and it hit within a tenth. This second time I corrected the control points the horizontal was within a tenth but the vertical was off a foot.