Speedometer Malfucntioning, Help

Hi, I have a 2004 Buick Rendezvous CLX and recently my speedometer has been acting wacko…and continues to do so. Two to five minutes into a trip it will slowly increase or decrease in mph’s and randomly stop (or continue) as it pleases regardless of how fast or slow I am traveling. Currently I’m using my GPS as my speedometer. Several mechanics checked it and test drove the car and no one could figure it out. The devices they used to check the speedometer indicated it was functioning properly. Two mechanics suggested asking the “car talk” community as someone may have come across this dilemma and may suggest a solution. So, here I am. Thank you for your time.

Duplicate posts are not necessary.