It certainly sounds as if the problem is the machine!
Personally, I would get an Innova with lightning stitch. Innova users seem to be the happiest longarm quilting group, and Innova provides 24/7 phone support. There are a couple of Innova groups at
http://groups.yahoo.com you could join that might help you decide on options. Here is the Innova website:
http://www.innovalongarm.com/
Granted, some tension issues are possible on any longarm. I just think they are less common with the Innova, and also more easily fixed when they happen because of Innova phone support.