PLEASE HELP! Slipping transmission...

Hi there! I need help! ahhh! My transmission has been slipping in first gear, but not all the time. I've had it looked at, and it seems it doesn't do it when they take it for a spin. I had it cleaned or whatever as the auto shop I went to doesn't like to do flushes. My car has about 88k miles on it. I bought it brand new and have had everything done as I am supposed to...at least I think. I have no clue what I'm doing with cars and need you guys/gals help that do know about cars. SUGGESTIONS PLEASE???? THank you!

Welcome to AF.

It would help to post the year and model Grand Prix.