I'm having issues with my ac on my C300. The ac works perfectly fine when I'm driving in the city but after about an hour of freeway driving the ac no longer cools and the airflow is reduced to almost nothing...at full blast it blows the same as if it were at the middle setting. If I put it in full blast, you can hear the blower working but there just isn't any cooling or full airflow. Once I turn off the ac and stop it seems to drain then the airflow is fixed but it won't be as cold as in the city. I have stopped sometimes when I have the issue to pump gas and buy something from the store and noticed the ac didn't drain and the airflow won't return. I'm hoping to at least have an idea of what is wrong so I won't be paying to replace a bunch of stuff only to still have the issue and return to have it services.

I have noticed that sometimes when I drive in the city you can hear the airflow increase a bit for a few seconds and then decrease not by much but I don't know if it has anything to do with my original issue. Any help will be greatly appreciated.