Dart swinger slowing down???

Pull the transmission dip stick and smell it. If it smells burnt (i.e. stinks big time) then you burned up the clutches and its time to rebuild the trans.

I had this happen the last race before the division finals a few years back (probably due to me launching the car in second gear all season...long story...). I had a friend rebuild it in a week, just in time for the race....good times.