Have you checked the arm that the feeder belt idler is mounted on. Sounds to me like this arm is binding and is stuck tight in about the middle of tavel distance. This would explain why your cutter bar slows down when you are cutting. The arm is not moving far enough to make the belt completly tight but yet it is tight enough that it won't let the feeder stop. I had simmilar problems on a 1688 I once owned.