Cavalier P0118 temp sensor problem?

My 99 cavalier has set the P0118 coolant temp sensor code about a month ago. Changed the sensor twice to be sure that wasn't it. Proved it today by testing the sensor itself. Sensor is fine. When I erase the code, car works great. Takes a few minutes for guage to work, then when it does, a few more minutes until its craps out again. this issue is causing the tranny to shift hard as well so you can see this being an urgent problem. All wiring looks fine, wiggle testing doesn't change a thing. Any answers or troubleshooting would be great. Thank you