Automatic seems to go into nutral as im just driving?

i just put a cat back on my car, and was told that my cat was clogged, im waiting to order a new high flow cat, but in the mean time i was just driving and my car felt like it was trying to stop it self as i was just coasting. and also it reved on its own almost like it was changing gears on its own. granted i know automatics do it on their own but i was just slow driving about 40 mph and the rpm's jumped sometimes almost to redline. then it went back to normal. later on in the day i was driving the same way and the rpms reved and i pushed the gas and rpms jumped with the gas but car would not move. then it came back and drove normal. got to where i was going and checked the fluid and it smelt like ****, and was like brown. i just had my rebuilt not even a year ago. my warranty is up i think, but i needed to get home and on my way home same thing but it was in (nutral) for longer so i pulled over and then started again it went and then did the same thing again. wtf