imprvn

I have a 2001 Lexus GS300. It is a US specs car but I live in Dubai. Basically it's an export car. I got it a year back everything was fine until this summer. But now my car's AC doesn't work properly. During the night time, or when I'm driving slowly (about 60kmph) it works just okay. But when I am on the Highway it doesn't work at all. Instead of blowing out cold air it blows out very hot air. I tried changing the AC gas but it didn't seem to work. If anyone could suggest what is supposed to be done in this case it would be really nice of them.

PS it gets about 50 degree centigrade in here. I sweat like a dog, someone help.