Have you adjusted tension according to this Jamie Wallen video?
http://www.youtube.com/watch?v=Q1mRhcquZTM
If not, I highly recommend that you try it. This technique works for all sewing machines, not just longarms. Basically you loosen the bobbin tension as much as possible first, then adjust the top tension. Be careful when adjusting that bobbin screw. I could not get mine as loose as he demonstrates; the screw fell out of the bobbin case first! Luckily, I saw where it went and was able to retrieve it. It's often recommended to loosen the bobbin screw while holding the bobbin and screwdriver inside a plastic bag. That way, if the screw comes out, you have a good chance of keeping it.
Also, what thread and needle size are you using? You may need to use a larger needle. For quilting on a frame, I find that it's helpful to use a much larger needle than I used when quilting on my domestic machine. Typically at least a size 16 is recommended, and I actually use a size 18. Larger needles are stronger and can take the speed and added stress of the machine moving on the frame.