W126 radiator over heat 90-100

[mercw126]

Hi...i need some help.My mercedes w126 280se radiator over heat is around 90 when driving on highway.first time i think maybe i turn off the a/c maybe the temperature will not going to 90c,but also the same problem.when i idle the car without the a/c the temperature will slowly increase to 100c.if i running again will slowly decrease to 90c again.Anybody can help me for this problem?Thank You