I know you said you cleaned the tracks but if you have a carriage did you check that? My carriage has some black plastic on it that has known to crack for other people. It hasn't yet for me but I keep an eye on that. Also make sure every single part of the frame is level. Your table and then your carriage and finally your machine. Something off in any of those areas can throw your momentum off. I was having issues just this weekend and under closer inspection I had a piece of thread wrapped around a wheel on my carriage.