As I drive I notice the engine revs real high; up to 3000 RPM as I change gears. If I come to a complete stop, it will rev at 3000 RPM and gradually (over a time span of about 15 seconds) rev down to 1500 RPM. Wheras before this problem, the car would idle at about 1200 RPM. Concurrently, the temp gauge reads 'cold' until the RPMs come down. Back and forth it goes like that when I'm not in gear or when I change gears. Is it just the sensor and is that expensive to replace?

 

Sounds like you need to change the ECTS and the harness that goes to it. Go to a GM dealer and not a parts store like Autozone.