When in 6th gear about 60 mpg and I speed up to 70 mph the rpm jumps up 2500 rpm than the speed catches up. Is this normal for these 3500 or is my clutch taking a dump? I am a Ford diesel tech and never seen this in the Ford. It only does it in 6th gear not the other yet. I just bought this truck a month ago and fixing all the little stuff on it. Like tappet covet oil leak.