I had something similar occur on my 00UTY a few years ago - air worked well for a while (between 30 seconds and up to 5 minutes!) then blew hot air.
I took it to a few places before my local Mitsi dealer arranged to keep it for afew days testing the air - as they couldn't fault it either. (compressor, gas, etc all ok)
In the end, it turned out to be a very minor issue - the auto clutch assembly on the air compressor had two slightly loose retaining screws. When the compressor clutch engaged, it wouldn't stay engage for very long at all, hence the hot air.
A few turns with the wrench - and the problem has never returned since! By the way, my air is running beautifully since new (apart from this minor glitch).
Has anyone had to get the aircon system re-gassed? Mine is 10 years old next May, with over 350,000Kms on the clock - and still using the original gas.
The air is still blowing at a measured 3 degrees celcius at the vents!