2000 GMC Sierra Switches gears

Especially on longer trips, 6 to 20 miles, transmission swithhes from overdrive back to 3 rd. While in drive. We stop turn off the engine and start off fine and it will switch back to 3rd gear ( eating up gas) This truck has been doing this for years, on long trips it's a pain. I'm tired of it and we are concidering a new transmission. It was worked on once while in Montana with work, they hooked up something to it, took it out for a ride, switch it on and it went into the right gear. They tried to fix it, costing a lot, but it didn't work.