Why is my 1997 ford taurus overheating?

I have a ford taurus which recently had a new radiator and heater core installed. Now when I drive it after a little bit of driving the temperature gauge redlines hot. The only ay to bring it down is to run the car at over 3000 rpms. When you do tht it cools off a bit and isnt as bad. I havnt noticed anything leaking out but have seen that my coolant level drops whenever i check it. Anyone know what may be wrong?