1993 Toyota Corolla Engine Temp

Hi, I just had the thermometer in my car replaced because it was faulty and the temp gauge was not moving. This was a problem because the computer in my car prevents it from shifting into overdrive unless the car is warm enough. As you can imagine, driving on the highway in 3rd gear is a HUGE waste of gas (5-6k RPM). The problem I am having now is that if I leave the car in the driveway for a while it warms right up, but then as soon as I start moving (mostly on cold days), the gauge drops to cold very quickly. The highway is down the street from my house and if the gauge is halfway up its about a 50/50 shot whether the car will stay warm enough by the time I get to the highway for the O/D to engage. Is there something else malfunctioning in my car, and if not, how can I get my car to think it is warm enough.I'm getting like 15mpg on the highway when it doesnt shift which is terrible for my car.
Thanks,
Justin