Might check that cable (can't remember what it's called) that goes from the accelerator linkages to the trans. It sounds kind of like how mine was when the cable was disconnected one time. It's just a thought, I'm sure someone that knows what it's called will chime in.

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1985 300D Turbo ~225k
2000 F350 (Powerstroke) 4X4, SWB, CC, SRW, 6spd ~148k
1999 International 4900, DT466e (250hp/660 ft/lbs), Allison MD3060 ~73k