Yaris TR 1.3 '08 AC problem

[Eeman]

Hi Everyone,
I bought a Yaris 1.3 '08 model (imported from uk to pak) and have been driving it around for 5 months. The car was in mint condition, it was perfect as long as winters were there. Didn't notice any problems until summers came and i had to use the AC. what happens is that as soon as i turn the AC ON, for a while it throws out chilled air bt as i drive on, the air gets warmer and it gets pretty warm. when i turn off the button and back on again, chilled air comes back bt after 5 mins or so it recedes again. I have had it checked from several local mechanics and this is what they did.

1. I had a freon re-fill as the mechanic proposed, the problem persisted and i had a 2nd re-fill which didnt solve the problem either
2. next time, the mechanic said there was a leakage in the hose so he replaced the entire pipe, which didn't solve the problem either
3. after changing, i had the drainage pipe cleaned several times but the problem persisted
4. Finally, it turned out the sea-air during import of car had corroded the filter and compressor, so I had it all entirely replaced.
For about 3 weeks it seemed to have solved the problem, But now it's back on again. The AC throws cold air for about 10 mins but as i drive further, the air gets normal and then warmer. Also, the AC does not perform v good when outside temp is v hot (above a 100 degrees).

NOTE: The car is imported so not covered under any warranty/insurance claims. I have already spent a lot of $$ on it n the problem has persisted, so i want to have suggestions on what i should look for now.

Much thankful for ur support!!