1993 Chevrolet G 20 Van Transmission

Hi, I have a 93 Chevy G 20 Van with auto trans. It is not shifting iinto 3rd and OD. The catalytic converter has been removed. I have flushed the trans. and have changed the filter. The speed sensor has been replaced. I can force it to shift into 3rd by stomping on the gas. When I do this the motor revs and slows and revs and slows but will only ocaisionally shift into third gear. It hits the shifting RPM then nothing but air. Any More Suggestions? Changing the speed sensor did nothing noticable. Thank you.