The Air-Conditioner quits when it is hot outside, now that the temp. has dropped here the Defogger keeps fogging the windshield up. When you start the truck it is okay but when heat starts flowing the fog starts and keeps going making it impossible to drive without a lot of wiping the windows and is dangerous. Could the system have a leak somewhere causing this to happen or maybe a bad heater core. We have had the truck for years but when we take it to the shop to have it looked at no-one can ever detect a problem let alone fix it.