Help C4 Problem

Hello, new to forum and ford automatics. Just purchased a mustang with C4 trans. With the shifter in drive it shifts great other than the early up shifts. Try to shift manually and the problem begins. When I go to 2nd it feels like nuetral. Engine revs to about 4000rpm then its like popping a clutch in 5 speed. Let the rpm fall below 4000 and its like nuetral again. Upshift to 3 and its normal again. Under WOT manual shifting works great and definately not sluggish.Thanks for the help in advance.

Can you find out what kit was installed (if it was a kit) or what mods were made to the valve body?
It sounds like there is a problem in manual selection of the gears. if it shifts ok in drive then drive it there and when racing you can shift manually. It's not a fix but it does by-pass the problem until you can sort it out.