I have 97 luminia. My car heater is not getting as hot as it use too. Normally it takes about 10 minutes when it's about40 degrees or below for my engine to heat up. Now the temperature gauge will go to normal ( in the middle) I turn on the heater and it will push out slightly warm air them the temperature gauge will spike up as if it's going to overheat and the heater goes cold then the temperature gauge will drop down like its cool. It will consistently do this as long as I have the heater on. After at least a good 35-40 the heat will kick in and stay warm. What could this be