700r4 issue 2-3 shift

[Wet..Paint]

When I first got my truck, It had a weak 305 in it and the trans was factory and felt like it was slipping into 3rd. After I put the new built 350 in it and I added a shift kit to hopefully help the trans. But now it seems like its missing the 3rd gear, then grabbing it. No idea how auto's work, but it feels like it shifts out of 2nd for a bit, then shifts into 3rd, and when it does grab it it grabs it hard. It doesn't feel like it slips at all, feels like it shifts out, motor revs up, then grabs 3rd.

I have researched it a bit but all I can find is slipping issues? What would this be? Should I rebuild it? Servo? Do I need to pull it apart to change the Servo? The most I ever did to a trans was, converters and shift kits..

Thanks in advance!