mondeo 98 2lt auto gearbox with no 2nd or 4th?

I have a R reg 2lt ghia x auto, it revs to 5000 rpm before it will change gear and then jumps straight to 3rd. It will not change to top gear at all and when i press the o/d button the light on dash works but nothing happens at gearbox, also after about 10 mins of driving the o/d light on dash starts to flash. The oil has been checked and level fine. Will i need a complete new gearbox or is it just a sensor?