The main thing to check first would be is to see how closely the rear paper table is contacting the back plate.  This is usually the cause of the trouble.  You may have to either adjust the table or the back plate--most likely the table.  If everything is ok here, look all around where sheet metal is in contact with moving parts.  After that, you may need to drop a tiny bit of oil onto the escapement wheel or the carriage rails.  If the carriage is loose, the rails will need to be adjusted inward a little.  There are, depending on the length of the carriage, at least four large bolts at the top rail.  Undo all of these and bring them down, with the carriage in the middle, as far as they will go.  Don't try to force anything.  Then try the carriage travel.  If it binds in either direction, you will need to re-adjust on the side that's binding.  It will probably take several tries, but eventually, everything should turn out ok.  --This is an older post, so in addition to attempting to help the original poster, I also want to help anyone else who has this particular problem.