2000 Continental overheats but only after driving 15 minutes can idle all day long at normal temp

Even when at idle the amount of heat generated when an engine is higher in the RPMs and under load is a huge difference. When you get a chance after it has reached operating temperature, grab the upper radiator hose and see if it has pressure and is resistant to be pinched cloed with a slight grip. . When engine is cool check level. You might try getting a new radiator cap if you have a temperature gauge and it does not indicate an aoverheating situation.

I am giving you a walk through diagnostic that will cover all the bases and go into better detail and walk you through tests and symptoms.

Here is the link;

https://www.2carpros.com/articles/engine-overheating-or-running-hot