Could be a tiny bit of fluff is caught between your upper tension discs. You can "floss" the discs with ***unwaxed*** dental floss to clean that area. (You do not want to deposit wax on those discs!)
Also, have you cleaned underneath the needle plate? (I don't have a Pfaff but assume you have a needle plate covering the feed dogs. There may be just enough lint compacted in there to affect the timing.