Air cond issue

[coriolis]

Just got back from a 3 days work visit in JB, and there's an issue with my aircond:

During normal city driving or idling, the air cond works fine and cold. However, when I drive on the highway at constant high speed (130+) beyond an hour, the air cond performance drops drastically. It's as if no air is blowing out at all, let alone cooling.

I usually set my air cond temp to 23~25 degrees, and when the air stops blowing the external temp is shown as 26 degrees.

Been searching the net for similar case, but usually its the other way around (hot at idle/slow and colder when speeding). Mine's reverse case :(

Anyone can give insight?

update:
was driving it around this morn, and tested the aircond a bit. Still works like normal and blows cold air, then when I set the temp to 1 degrees less than outside temp, the air that blows out was noticably warmer. Same goes for when I set the temp equal to outside temp.

Maybe I should set the inside temp way lower than 23 when I'm on highway at nights. :/