Make certain you are using the correct needle and it is properly sized for the thread being used. The Pfaff GQ 1200 and it's siblings (Viking MegaQuilter and Janome 1600P, all made by Janome) use special high-speed needles. They're labeled HLx5.
If the top thread is breaking, you may want to reduce the number of holes being used in the 3-hole pretensioner. With Gutterman 100% cotton, I use only 2 holes. But with other threads you may need to use only 1 hole. And be sure to use good quality thread.
Also, you may want to set the speed slider to the middle setting, which lowers the top speed of the machine. These machines are capable of running twice as fast as our normal domestic sewing machines at 1,600 stitches per minute.