Transmission slippng

Hello, I have a 1999 Honda Civic LX with 165000 miles with an Auto transmission. The other morning while driving home from work the car just slipped out of gear while exiting the highway and would not go back into gear. The transmission was still in drive and it appeared to be in neutral. While pressing the accelerator the engine would just rev up while in gear. After the car set for an hour the transmission went right into gear and drove about 1/2 mile normally before it slipped again. It is like the transmission just goes into neutral on its own. The fluid level is normal.