Cadillac Repair: engine overheating, radiator cap, head gaskets

Question
2000 Cadillac Deville 32 valve Northstar engine 73K miles.
Engine overheats on 25 minute ride home using air conditioning.I took the car to the dealership twice. Both times they tested the radiator and said it was fine. One of the mechanics took the car home and sympton did not occur. The engine has never overheated before, even after a 7 hour trip using the air conditioner. Now it's almost a daily occurrence after my 25 minute ride home. Off course the extended warranty period is over.Any ideas or suggestions? Thank You

Answer
Are BOTH cooling fans operational ?.

Could be a faulty thermostat.

Could be a leak in the head gaskets of which need to be tested by applying constant air pressure into the spark plug holes for each cylinder one at a time and see if you see bubbles appearing in the radiator with the radiator cap off.