Temp sensor error?

My jeep has been with out a thermostat untill last night . It has been runing a little over half way up the temp gauge but when I install the thermostat it wants to run hot acordin to the gauge. The motor shows no sign of over heating the water pressure doesnt get heavy enuff to trip the 13 pound cap , the motor doesnt feel that hot. I relieved the pressure and took a look the water didnt boil and it looked to be flowing ok. SO Im thinking the gauge is wrong , its a new gauge and I have a new sensor with all new wiring inbetween. Any Idears

 

does the temp. gauge creep up or does it show hot as soon as you turn the Jeep on? could be a ground problem with the gauge

 

Creeps up like its working but I did noticed it was still reading about half way up a hour after I cut it off , Im going to check it closer today to see if its floating a bit above the cold mark before I crank it up.