Transmission tie up?

I was told this is what my problem is. I have a reverse manual valve body 727 trans. When I shift from 1st to 2nd the car wants to nose dive or dip down initially before it goes,almost like temporarily stepping on the brake, 2nd to 3rd is fine. Installed a new shifter cable,old one was stretched and rusted, but still have the same problem. When the car is cold the problem is extreme and the car really noses down, not nearly as bad when the trans is warmer, but still there. Any thoughts?