i have a stich problm?

i have a seiko jw-28bl-30 timming is set to the booke (oh im a mech on sewing machines) but nevery had the following prolbimg every so often i get a stich that actualy hangs of the feed dog leaves to thread hang after you pull the material out but the stich is not a skip stich i need help what wrong ive check timming needle bar lenth and connection belt timming all is good an a new hook to help e mail me at jonesracing6@wmconnect with your help please