Speedometer drops down on its own when driving

Hi

My husbands vehicle about three weeks ago got a new transmission replaced. Three days ago while driving on the highway doing about eighty mph when the truck on its own dropped down its speed to about fifty mph. It is shifts up and down on its own. The mechanic that replaced the transmission has no ideal what is going on.

Please say you can give us some direction!

Thanks

A wife of a H3 owner.