What weight is the Connecting Threads thread on the top? Even their 50wt is a heavier (3 ply) thread than the Aurifil 50wt (which is a 2 ply). I suspect your problem is with the balance between the top and bottom thread weights (or thickness).
You can try using a heavier weight thread than the Bottom Line (which I think is a 60wt?) in the bobbin, something closer to the Connecting Threads.
I also need to use a larger needle when I used Connecting Threads 50wt. I can use an 80/12 with Aurifil, but not with Connecting Threads.