The 2000 3/4 we have is missing after going down a long grade with foot off the throttle. When I got to the bottom I pushed the clutch in to stop and it was missing. I barley gave it some throttle and it smoothed out and ran fine. Fuel pressure 17 at idle 11 wot. VP and LP was replaced 300 miles ago. I watched fuel pressure while this was going on and it never changed. It did this before VP was replaced a couple of times. Truck is stock with muffler gone. What could this be??