2002 Hyundai Sonata Loses power/altenator ok, battery repla

While driving my Hyundai on the freeway, I started losing power, my radio shut off, spedometer wouldn't move and my battery light and other dashboard lights flashed on. I pulled into a gas station, turned off the car and it would not start up again. I have AAA, it was towed to a mechanic who ran diagnostics, said we only needed a new battery. They said the altenator was good. Got the new battery and when I picked up my car, went through a drive through and as I left I noticed I was losing power again. Took it to another mechanic, they checked the altenator, said it was good and not sure why I'm losing power, they charged my battery and I left. While Driving home I noticed my car surging and then I lost power again, I live in a remote location where it's a 75 mile drive to the mechanic. My cruise control is also not working, it will turn on and then it quickly turns off on its own.
Any ideas why this is happening? The mechanics at both locations said they do not know why I am losing power.
Both mechanics said that I will probably need to go to a dealership, however the closest dealer is a 3 hour drive. I am also worried that they will try to take advantage, I'd like to be prepared with as much info as possible so I don't get scammed by the dealership.