1999 Ford Taurus Transmission

If I'm driving 45mph or higher it works fine, but if I slow down and try to speed back up, it gets stuck in a lower gear and I won't be able to get above 25/30. Sometimes it'll jump back up eventually but I usually have to pull over, turn it off then back on and try again. It's getting to where it starts acting up any time I'm driving at a lower speed and the speedometer and odometer sometimes go out when it does this also. I've already checked the transmission fluid which is at a normal level. Any specific ideas as to what may be wrong with it would be greatly appreciated. I assume it's the transmission.