Whis does my tranny kick out of gear on 1999 chevrolet silverado

I was having trouble with my tranny shifting real hard from 1st to 2nd for a good while but the problem was always off and on. Then one day I was pulling a boat down the hwy and it began to start shifting in and out gears and the transmission is hot light came on. I bought a rebuilt one soon after that from a place near by that was known for doing good work. I moved across state about 400 miles away 2 weeks after replacing it. After I was here for about a week or so it started kicking out of gear and going to neutral on me at about 15 to 20 mph. Me speedometer would jump all around too while this happend. I took it to the local mechanic here and he tells me that i need to buy a new one or have mine rebuilt. I dont know the guy at all and dont know if hes the truth or trying to get sum money out of me. The tranny is under warranty but the shop is 8 hrs from here and i have no way of getting it there. Would a tranny go bad enough after 500 miles on it that it needed rebuilt again or could there be something else that I might could try first?