I can assure you that your decision was right. The older Bernina will still be stitching along years after your $300 modern machine. Most have metal parts and if you treat it right, you will be sewing on it when you are a gray haired old lady. I think beginning sewers get so discouraged using the "throw away" machines of today. I think you made a great choice.

I had a similar problem and figured out I was bumping the stitch button with my arm and the stitch changed.

Older Bernina machines are great. I bought my used and it is about 25 years old.