1999 Chevy Monte Carlo - What could it be?

I drive a 1999 Monte Carlo with 69,000 miles on it. Recently, my car started to "slip" right around 45-50 mph where it shifts into overdrive. It feels like when you spin your wheels on ice and then catch some solid ground off and on. Once you get above this speed everything is fine and it also doesn't do it until you reach this speed. If you accelerate with the car in Drive instead of Overdrive, the problem doesn't occur either. I changed the transmission fluid which was very clean, nice and red without any metal in the bottom of the pan.
What could be causing this to happen?[/quote]