Really sounds like a fluid level issue. Just do the correct procedure to test the fluid but do it first when the car is cold see what the level is. Then take the car and run it on the interstate for a little while. Then go home and while its still hot test the fluid level. Getting the level just right is a little bit of work but its a must on the auto tranny. As you drive it heats up and thins out. Thus you end up with more fluid . Also did you replace the filter when you changed the fluid? could be an issue with the fluid having trouble passing through the clogged filter.