When my TA heats up the gauge is ok untill around 170 F, after that it starts to jump around. Today I decided to check if I had an air in the system and did not put the radiator cap on. The gauge went to around 170 F and stayed there. After some time I put the cap on again and then the gauge started to rise and jump around.

I am sure its the temp sending unit and am going to replace it.