irishrose,
Some of the foot controllers have contact points. If they get dirty, corroded or burnt it will effect the way the machine runs. I've fixed a couple just by cleaning them.
One of the other styles has a carbon pile in it with some adjustable contacts. I've never had any cause to work on them. Just cleaned them out and occasionally rewired them.
If the machine starts running then slows down, I'd be looking at the motor first. If you have another one to try I'd do that.
My Mr Crinkle's busted motor that I glued back together ran OK but was loud and rasty sounding. Then it started slowing down. I couldn't find anything wrong inside but there must be.
Joe