My TS 55 started out perfectly perpendicular but gradually shifted. The problem is that the blade is not perpendicular to the base. You need a small high quality square to check/adjust the squareness. I now check it regularly.
As a further note make sure that your rail is supported along it full length by a piece of wood the same thickness as the one you are cutting. Any twisting in the rail can cause this problem also.
Supporting the rail is particularly important with the TS75 which is heavy. The guide post at the front of the table is for alignment, not sufficient for support especially if the workpiece is relatively narrow.
