Why causes a transmission to arbitrarily slip out of gear particularly after slowing down from the upper gears and then trying to speed up again - am I avoiding the inevitable of a new tranny?

More Info...to give you a background. It's a 95 Chrysler LHS, auto trans... the engine has been having some problems, which might have been taxing the transmission. It just recently started the slipping. I would be fine when first getting on the highway, and then after coming off the highway, i couldn't get above what appears to be 3rd gear. And sometimes if it revs, and then settles, it'll pop into gear. Also, pulling over, and turning off the car seems to reset everything, and I'm good to go for a bit. I brought it into my "trusty" mechanic and they say there's pretty much nothing they can do, short of a new trans, which at this point isn't worth the car. Apparently it's controlled by the computer, so nothing else can be done. (is this true?) The fluid is fine, so i don't know if there's a patch out there that could restore it to temporary proper working condition. I do mostly highway driving, but getting to the highway has become nearly impossible. ANY help is appreciated - spothaniel@gmail.com if you have any questions.