That is so odd! My Janome started doing the SAME thing today. Is it a quilter's full moon? I cleaned mine from top to bottom, changed the needle, held my tongue right and it still kept sewing on its own. Mainly just over bulky seams. I'm going to keep plugging around with it today and try to keep my fingers out of the way.
Maybe the dealer will offer a suggestion over the phone versus giving my "baby" up for any amount of time. I am getting a "Fc" (foot control) error after it happens, so maybe it's just the foot control? I'm a complete newbie with machines. I've only had this one since October.