You could pull the sensor, and put it in boiling water (100 degree C) and see if your gauge is correct....but I bet it is ok.
If when running down the highway you're at 88 or 90 degrees C, but heats up at slow speed, are you sure that the new fan clutch is ok?? You should be able to tell if the fan is locked up at the higher temp by feeling the air flow FROM THE FRONT OF THE RADIATOR. BE CAREFUL, as we need you to have all your fingers to post on this forum. At the higher temp on the gauge, the fan should be pulling air through the radiator.
I had a toyota that would heat up at idle but was fine at speed...turned out to be a bad vacuum advance unit. Your car is gasoline...correct? If the vacuum advance is too far retarded at idle, the engine will run hot at slow speeds. It only took me about a year to figure out the toyota, after, of course, I changed everything in the whole cooling system. If your car is gasoline, check the ignition timing at idle to make sure the vacuum advance is working at speed and idle.
Hope this helps....be careful..