Transmission popping out of drive when taking off at a starting position?

Recently my car after driving to Vegas popped out of drive into neutral when just starting out at a light. I was able to make it home two days later without any problems. But once again the next day after driving on the freeway for about 30 miles I was in traffic in the city in stop and go traffic when it popped out of drive into neutral again. This time I changed the fluid and added some Lucas seal reconditioner, but it did not help. Since the problem has become been happening more consistently, also when coming to a stop more quickly than usual it will cause the transmission to stutter and not engage lower gears, almost stalling instead. Just yesterday when at a light I began to step on the gas and the car did nothing even as I went through all the gears not until I put it in park and then back in drive did it perform normally again!