I too purchased a used Pfaff GQ 1200. I also got the Grace NextGen frame with my machine.
Have you checked/changed the needle? Perhaps the needle is the incorrect size for the thread you are using. A needle too big or too small for the thread size can cause breakage.
I was able to sew a number of practice pieces and an entire queen quilt without a single thread break on mine. For the quilt, I used YLI invisible thread in the top and Gutterman variegated 100% cotton in the bobbin. For the practice pieces, I was using Gutterman 100% cotton in both, but a different plain color.