727 surging in 1st and 3rd

My buddy has a non lock up 727 that has a surging issue in 1st and 3rd gear, if you're driving at a steady pace it seems like the trans is slipping then hooking up. We put a cheetah partial manual valve body in it and it didn't fix the problem. I'm not a trans guy whatsoever but if like to figure it out without having to pay someone. Any help is appreciated