I start the engine the temp gauge goes 0 , after running for a while the gauge will move up to showing warmer temp, then it goes  like it was before starting, I also have another gauge the sending unit is next to thermostat, it works great. You think it may be the sending unit checked all the connections they seem to be tight 

If the gauge is original or the original type it should read hot when the ignition is off, then go to cold or some value between cold and hot when you turn the key on, depending on whether the engine is dead cold or warm. Since it works for a short time I'm inclined toward a loss of either power or ground in that circuit. You need to check with a volt/ohm meter while it's acting up. I'm inclined toward the sender going open internally.