I know you called your service guy, but did you call the frame company? Mine has a customer service tech that will help us troubleshoot over the phone. My dealer has changed twice since I bought this system, but I can still call the company. Hang in there! I had tension issues off an on for a couple yrs. Service tech walked me thru troubleshooting a couple times. Then one day, the bobbin winder died. (I really did not like that bobbin winder..and I bet it knew that too!) Bought a used one from an online friend....other that usual 'life with a long arm', I have not had ONE tension issue since. You will figure this out!
PS you might like Machine Quilters Resource. Its a website for machine quilters. Could be there is a quilter over there that has had your same issue.
www.mqresource.com There is a machine maintenance thread if you scroll down to near the bottom of the forum list page.