Considering its a 99 GTP... normally they are well over 100k mileage and are getting tired. You said "new gtp"... new to you means you just bought it.... most likely has had a tough life and previous owner beat on it and the tranny is just tired.

Simply race to 70mph... if you aren't winning by then you need to stop or call it a tie.

Besides, street racing is illegal, highway speed limit is 70mph in Kansas, and if you tracking the car you need a new transmission before you go back (or make sure you have a friend with a trailer for when the tranny finally says F* You and dies)