You might want to join the groups specific to Nolting machines. Here's one on Yahoo!, but I'm sure there are others too:
https://groups.yahoo.com/neo/groups/..._quilters/info
Have you watched the Jamie Wallen Youtube video on how to set tension on a longarm? If not, I would definitely try his method of loosening the bottom tension as far as possible, then experimenting with the top tension until you get a balanced stitch:
http://www.youtube.com/watch?v=Q1mRhcquZTM
If you haven't yet done so, run your finger all around the edge of the needle hole, all around the bobbin race, and all around the bobbin case to make sure there isn't a little nick or burr someplace (maybe caused by the broken needle hitting it). If there is, smooth it out with some ultra-fine sandpaper (jeweler's paper). It may be that the thread rubs on a nick over and over only when you move the machine backwards. Also rub your finger on the machine all along the thread path above, making sure that the machine's outer case is not nicked and that the thread guides are smooth.
Take a good look at the needle and make sure it is straight, not bent in any way. Also make sure it is seated correctly for your machine. (Different machines have different quirks about that needle seating.)