Trans going crazy! But then its fine?

I was going around 75 on a highway in cruise. All the sudden the car made a noise and the speedometer says i'm going 60 and its still in criuse. So i take off criuse and give it gas. Giving it gas made me almost crap my pants. When i gave it gas it would rev like it was in neutral and the speedometer would go up to 80 without the car going 80. I pulled on to an exit and found myself almost coming to a stop even though i was giving it gas and by giving it gas it would make a hissing whinning noise and wouldn't go any faster.

After driving around and getting back onto the highway i noticed everything was fine, I was going 80 this time without criuse to help and got back home safely and today my car is running like nothing happened.

It has a 5k rebuilt tranny on it 190k 1992 LSL and was pampered by the original owner I am the second owner. Anyone know whats going on here?!