the car runs at normal temp, so says the gauge in car. the upper radiator hose is hot, the lower radiator hose is cold.also if i take the pressure cap off of the coolant reservoir, the liquid doesnt go down as the car runs, but goes up and starts to overflow. Is this a thermostat problem, or what could it be?

since the reservoir tank is overflowing, isnt that a backflow since maybe the thermostat is closed

 

The thermostat sounds like its sticking in cold engine mode. Is your idle elevated to about 1200 RPMs?