my transmission slips when cold. do i need a rebuild?

if i accellerate normally, it shifts from 2nd, to 3rd for a fraction of a second, then straight into o/d. unless i step on the pedal harder will it go into 3rd. the fluid level is good. thats why i am hoping it isnt the clutch plates ot torque convertor. if i step on the pedal hard, to pass and downshift, it might go into 2nd at high rpms and stay there till i release the pedal. then it will drop right into o/d. if i hit the pedal EXACTLY right on, it will go into 3rd so i can pass, but thats rare. anotherwords, it doesnt shift correctly anyway. this is before it started slipping. i dont dare try it now.