what causes the motor to overheat on 1999 Cadillac DeVille

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on the freeway cruising 62mph, the car runs fine, however when I slow down the all hell breaks loose and the car overheats. In the the city, the car overheats after 30 minutes driving.
(2) Answers
Best guess : radiator cooling fans are not working. Ram air at speed will cool rad. but at slower speed fans have to do it.
most likely the cooling fans or its circuit...this vehicle has a complicated cooling fan electric circuit.