T_risker

I have a freshly rebuilt 4l60e because I was having problems with my 1-2 up shift after I apply moderate to heavy throttle. I have a stage 3 cam with heads it's only making 311 hp so I shouldn't be making too much power in my opinion it's a tuning issue I'm just trying to confirm and get a second opinion on what I should tell this guy... He keeps telling me it could be a faulty tranny which is truck except they are tested before they leave the facility they are built at...... And I can also manually shift it starting at first and it shift how it should...... Please any input is appreciated.