I was sitting at tacobell tonight, waiting for my food...the road at a slight upward tilt..I noticed my temp reading close to hot...I shut down my car cause I was afraid of over heating. When I started my car and drove off the temp went back down...I was a little puzzled...i floored it a lil, no change it temp..I went on another hill and stayed idled...no change in temp....I came home and saw that my resevoir coolant tank was still full....

Could this be a thermostat problem in the making?

 

Probably either Thermostat or your fan.