700 R 4 tranny shifting problem

My son and I rebuilt his 700 r 4 tranny which is a 1989 core. We installed all the goodies (New kevlar band, new frictions and metals) along with a new torque converter and a shift kit. After we installed it, he has never been able to get third gear wide open throttle. So, we installed the oem governor which shifts at 4500 rpm but still no luck. We grabbed a new/used valve body, drilled the proper holes for the shift kit, and still no luck. If he backs off of the throttle for a second at 7000 rpm, then he can get third. What might be the problem? Thanks in advance.