Jeep Repair: A/C blowing hot air, jeep cherokee, grand cherokee

Question
I have a 2001 Jeep Cherokee and the A/C blows hot air when it's warm or hot outside, but blows cold air it's cool outside.  I took it to the dealer and he couldn't find anything wrong with it. Help!

Answer
grand Cherokee   or baby Cherokee??   last 8 of the vin will tell me lots on what kinda jeep this is. Did the dealer do anything to it  or just said they dont know?  ANd can u give me some more detail to when and how the ac works for u  such as does it work for awhile then stop blowing cold air and then u can restart the jeep and it works again after it cools off?