98 Cadillac DeVille overheating

My 98 DeVille started overheating because it was 3/4 gallon low of coolant. Since, it slowly works it's way up to 245 then comes back down to 212 if you turn the air off. I have found a leak. There's no water in the oil. Is it likely that the Cadillac service mechanic can diagnose it?

Thanks

I meant I haven't found a leak. If it's a head gasket I'm getting rid of the car. I'm hoping it's something else

Had a 2001 mercury sable doing the same thing loosing anti freeze every two to three weeks and no visible signs of any leaking. Put in some Aluma Seal stop leak into top radiator hose after removing it from radiator because had no radiator cap ( could have put in coolent tank but would have taken longer to work ) after that no more losing anti freeze keep the vehicle for another two years and still was not losing anti freeze when I sold the vehicle.

It doesn't overheat when the air is not on. And sometimes I can drive 10 miles and it never goes above 196. But sometimes after 10 miles it slowly gets to 45th when the air is on. Turn the air off and it goes back down to 212