I have a 96 Passat TDi with 140k miles on it. It runs flawlessly at speeds up to 85 mph. If speed is 65 to 70 and the road begins a gentle climb, the engine rpm slows down 5 - 10 mph even tough I try to give it a little more fuel. What could cause this? I've been consistently getting 46 - 47 mpg up till now. Any ideas???