I own an '02 V6 Mustang
with the same problem. I changed the transmission fluid a couple of weeks ago and it's still happening. I just bought the car in November, and it still has a warranty on it, so I'm planning to have it checked out soon.
I'm hoping the transmission isn't going to drop out of it. It already had high milage when I bought the car, and I worked as a merchandiser and put a lot of miles on the car in the past 6 months.
Other than it slipping between 40-50 mph, the car runs great.