Sounds as though you have a mechanical issue
Look inside the rails that the carriage rides in and see if anything is stuck in them I had a customer that made ceramic tile and once a month I had to chip clay out of the channel mind you not a big piece a very small piece that was pressed into where the roller would go and that was enough to do that it did same thing the piece was smaller than pinkie nail but it caused a lock up hard to believe but most mast have very little clearance between rail and carriage bearing also check your carriage bearings and be sure to be safe when working on the mast chain it and double chain it they can kill you !