Transmission issue HELP!

What is your question?

So while driving after an hour the tranny seems to go into standby mode? Drives completely fine.. Drive on the freeway and take an exit, downshifts and then won't shift after that til the tranny cools down. Bout 30 minutes later car drives just fine til I've been driving awhile and does the same thing once I slow down. Been in 3 different shops and they can't figure it out. Was told to get a new tranny but would the tranny be blown if I can still drive it?! Any help would be appreciated! Thanks