My car is a 2000 Grand Prix GTP 53,000 miles

My trans shifts very firm, And doesnt give me crap, Ive had it for a week or so now.

Today i noticed 1-2-3 shifts perfect than i noticed when going into 4th (overdrive) Its rpms jump maybe 200-300rpms than come down quickly and it locks up and stays locked (this isnt every time just depends on throttle position), It doesnt slip out of overdrive but does jump a few hundred before settling down, With it having super low miles i really cant understand it having a bad tranny?

any help would be appreciated


Nate

(Pic of car)