I agree with Dunster. It sounds like your rails are not equal distance apart the entire span of your rack or that the rack is not 100% perfectly level and square the entire distance. You can try measuring the distance between the front and rear tracks. You have to measure everywhere. One way is to cut something rigid, like a dowel or piece of pvc pipe the proper distance then slide it all the way along your rack between the rails that the carriage rolls on to see if there is a place where it is too far apart or too close together. Or tell your service guy you think the frame is out of square or out of level or one of the rails is somehow not right and you want them to come check it and fix it.
I chatted with one longarmer the problem was in her subfloor. It flexed and caused the rack to be out of skew. Usually when it feels like the machine head is fighting you and it happens in the same place it is an indication the frame is the issue not the longarm machine head. And yes a computerized machine would also have problems with an out of square or out of level frame.
My longarm I have to relevel it and check it annually. Houses settle and some move more than others with the change of seasons.
I seem to recall you had a similar issue with your used HQ and I offered you the same advice then. (that your rack or setup was not perfectly level and square the entire distance of the rack)