1997 Acura cl overheats when outdoor temperature is over 80 degrees

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Radiator has recently been flushed. Thermostat replaced in 2014. No leaking from radiator. Temperature gauge goes way up when outdoor temp is over 80 degrees. 1997 Acura CL 3.0 mileage 128,000. What seems to make the problem better or worse? Better after driving awhile and when outdoor temp is below 75 How long have you had this problem? A few weeks