In my 70 pick up I have a fuel injected 302 with a AOD trany and 3.25 rear end gear.
It does that jumping up and down in gears at about 50 to 55 MPH.
It's a dog all the way around in the performance class, so I keep it in drive till about 60 MPH then I pop it up to over drive. Now keep in mind this truck weighs over 5 thousand the way it sit's. Now this thing can run top end pretty good as long as there is no hill or a strong head wind. If there is a hill or a wind it will get to about 75 MPH and won't pick up any more speed. So yes this is a big dog, but it was this way when I bought it.
The motor does not have enough torque with those gears and trany to pull strong.
The only good thing is the 30 plus miles to the gallon it gets.
Kurt