usually if my thread gets really tight it is because either it is wrapping around the spool holder (under the spool) or is threaded incorrectly- maybe wrapped around something . it must be catching on something. if it is heavier than what you have been using you may need to loosen the tension a little on top- but it sounds as if it's catching/wrapping on something. good luck i hope you can figure it out. you should be able to use many different weights of threads without this happening. if it continues you may want to call your dealer & ask them for a hint/tip- advice. if you are using cones what kind of cone holder are you using- could it be catching somewhere traveling from the cone through the thread guides?...just trying to guess possible causes...