2008 Ford Ranger: tranny only slips when warm

2008 4 L 6 CYL , 133,000km When tranny is warm, it slips in drive on take off, when I slip it to 2nd or 1 st , it takes off from stop, no problem, no slipping at all.

I asked a tranny guy, he says valve body kit replacement, sometimes works before rebuild…what is the probllem??