Idling Temperature Problem
I am chasing down the same problem. I have found that my electromagnetic fan clutch (for the main fan) does not engage until about 110 degrees or more. It should close at 100 degrees. When it does go on, it easily cools down the car. I verified that the clutch does operate by shorting out the cable connections that go to the temp switch.
I verified the operation of the temperature gauge by noting the temperature at which the car stabilizes after warm-up. I just replaced the thermostat, which was marked as 87 degrees C. That is just about what the gauge registers.
I have ordered a new sensor and expect that this will cure the problem. Do some simple checks to verify that the EM fan clutch does operate. Look at the temperature after warm up. Derive your own conclusions about whether the temp switch is operating properly.
Good luck. I will post here the results after I install my new temp switch.