Transmission slip...I think?

[digem250stroke]

Having my powerstroke for quite some time now I have more or less done all the repair/mods myself but I would never have gotten this far without the help of all you guys and your knowledge on this forum. Thank you!

At highway speeds, If I let off the go pedal and let the rpms get down to around idle and than re-apply the gas, theres a sort of dead area in the drivetrain. i can get the rpms up to about 1600 but with no forward momentum. sometimes if Im climbing a hill, I can hold the rpms there and still feel the truck decelerating. Once I push the rpms up to about 1700-2000 I can feel it start to push the truck along.

Has anyone experienced this before or know what could be causing this?
Any advice would be greatly appreciated.

p.s. sometimes when the truck is cold on the first run down the street the rpms will run up for a sec and then re-engage on its own at around 30-40mph.