My Janome 9000 also liked to sew anything from a few stitched to several inches. When it was returned, the dealer said all my machine needed was a good cleaning. He also cleaned the foot control. Brought it home and the extra stitching continued. -- I never was impressed with this service person which was confirmed by his knowledge or lack thereof. Urgh!!! Good service is so important.
I had purchased this machine from eBay store. I called and talked to them and was told it probably was the mother board and if I could, to unload it. I did. I know have a new "used" toy - a 450 Bernina with stitch regulator.
Those of you who have a knee lift, GET WITH IT. They are wonderful.
Suz