longarmlizzie I'm sorry to hear that you've had such horrible issues. I tell you what tension is a nightmare on any longarm. I've learned the hard way that you have to be part machine mechanic when you own your own big machine. They are finicky at best but once you learn to get along they're great.
I had horrific tension issues. Every single bobbin I wound while I was quilting turned out to be totally different from the last. I heard about Glide thread and tried it and have not looked back. I don't need to readjust tension even when changing colors up or down. It's awesome stuff. I use a 16 needle with it and it works brilliantly for me.
Have you seen Jamie Wallen's tension video? It saved me from seeing if my machine could fly from the second story window...
https://www.youtube.com/watch?v=Q1mRhcquZTM
I would also suggest joining the Tin Lizzie yahoo group. They're really helpful. Next time before sending your machine in to be fixed you might want to ask them. Most of the stuff that can be tweaked in a shop can be tweaked at home. Resetting your timing is easy once you make friends with it. It took me a long time the first time but now I can do it in minutes.