transmission problem

The engine revs up, and the tranny shifts down to first....as I'm driving. I've been stuck on the highway. At first local dealer said it was a faulty shift assembly, said I spilled water into it, and replaced that. Then said it was the transmission plate, and replaced that. Then said I had spilled water into the shift assembly again---I hadn't, and haven't had any drinks of any kind IN the car--and replaced that again. Now, after almost $8K of repairs over the year during which this kept happening, the dealer NOW says.....there's something wrong on the connection into tranny (? not sure what that means), and that to find the glitch will take so many hours of high priced labor that it's better to just replace the whole transmission, $7K.....even tho they don't think there's a problem with it. I've babied this car, kept up on all servicing, repairs, etc....and this is the transmission that is supposed to be problem free. Any ideas? comments? heard of this anywhere?