I am still learning all the idiosyncrasies of my machine and learned a good lesson last night. I had started quilting a new baby quilt on my short arm machine (Pfaff Grand Quilter) and it kept breaking the top thread. I finally realized that after the last lap quilt I had not lowered the rollers so the new quilt was sitting about 2 inches above the bed of the machine. Evidentally my machine does not like that. So I lowered the rollers to just above the bed and I haven't broken a thread since.
Hope this information will be of some use to some other frustrated quilter. Also, to prevent thread clusters under the quilt, make sure you LOWER THE PRESSER FOOT.