I suspect you have a similar system to the turbines. Basically, the hot bleed/supercharged air comes into the air cycle machine. On the ground there's a fan that supposed to blow air over the cooling fins to cool the incoming air. In the air the cooling is achieved by the airstream blowing past it (normally there's a scoop or similar at the top of the fuselage behind the main wing). What happens is that that fan and fan motor in the air cycle machine is notorious for breaking down. So if it's first cool when the bleed air is cool, then gets hot on the ground and then blows cool up in the air again, then that's most likely what's wrong.
But I could be wrong - maybe you don't even have an air cycle machine and it's something else.