I had the same problem with my old Bernina Record (830). I went along for almost thirty years without a problem whatsoever. Then tension problems. I worked and worked at it and still it is sitting in the spare closet, frustration made me buy another machine.
I should pull it out and work at it some more. I loved that old machine and it was hard to retire it to the closet.
Hubby bought me a janome for christmas a year ago and it is a good little machine. I bought all the feet for it and some other extras. But, it wasn't a bernina.
So, you guessed it, I found a newer bernina artistia 180 with so many extras included, that I couldn't turn it down. Just a month ago. I just took it to the quilt store dealer to have it cleaned and checked out. I had cleaned it and fiddled with the computer tension balancing. No help, the tension was acting up. The tech said NEVER USE CHEAP COATS AND CLARK thread. It may save you money at the time, but, you will pay in the long run with problems like this.
I am going to throw every one of those coats and clarks away. I bet that was the only thing wrong with the Bernina Record. Oh well, live and learn. And it is nice to have the ability to use the embrodery module now.