Even though the system makes the times (including ball times) in tolerance, if the speed deviates a lot up or down ball to ball, it won't "feel" right. Now another thing to consider before you get to concerned. At 28 mph is the skier your pulling really skiing? The system adds RPM to counter the strong pull of a slalom skier than modifies the speed with GPS input. Your ball speeds show the system constantly bleeding speed. It could be the skier is not skiing the pass or is learning. Is this a possibility?