Another vote for cleaning out the old pedal, or getting a new foot.
My friend's machine started sewing all by itself. She tried several things but the one thing that fixed the problem was her husband took apart the foot and cleaned it out. It was full of dust and hair. She is quite the clean freak, so I can only imagine what MY foot pedal looks like, lol.
My machine started acting up - it would stop sewing in the middle of a seam. I tried everything I could think of, then I remembered that both of my Brother machines have the same, interchangeable feet. So I swapped them, and now my machine works great.