560 SEL Temp Problems

Have an 86 560 SEL, have just changed radiator, fan clutch and thermostat. Aux fan is working properly. If car is sitting in park with A/C on in 90+ degree weather, temp goes to 100 (line above 80 mark) and stays there. When I then take the car on the parkway. driving at about 65 mph, temp goes up to 110. After about 10 minutes of driving, temp gradually drops back to 100. If I shut the car off for only 5 minutes and then resume under the same conditions except for local driving temp gradually goes to 100. Upon entering parkway again, temp goes up to 110 again. When I get off parkway, temp goes to 100. Additionally, sometimes as I'm driving locally temp gauge will occasionally go up to 105, back down to 90 etc but does not rise when standing still but increases when moving. Could new radiator be defective? Bad sensor for guage? Any suggestions would be appreciated.