2001 Ford Taurus A/C System becomes blocked

In 2007, I had a local shop replace my A/C compressor and clutch. Ever since then, I have been experiening problems with my A/C system. It operates well during operations in which the rpms are constantly changing, however, after 30-45 minutes of freeway driving or idling, when rpms are constant, the a/c system will act as if it is blocked. The blower is operational and air is trying to move through the air vents. There is a clear sound of something blocking and it is coming from the dashbord on the passenger side. I've disassembled the blower motor and fan inside the dash and it works finde and checked the vent door inside the dashboard and it appears to function as it should

I've taken to the shop several times only to hear they cannot find a problem. I've had them test the vaccum system and no leaks were detected. This happens on all settings, A/C, vent and defroster. The only thing that temporarily solves the problem is to turn the entire system off for a period of time, usually 15 minutes or so before it will work again. I am not sure it this is normal, but the A/C lines inside the engine compartment will be covered in ice when I am experiencing this problem. I read somewhere on the internet that another Ford had a similar problem and I believe the solution in that case was either the high pressure or low pressure sensor needed replacement. Its driving me nuts because its starting to get hot and I want to fix my A/C know what to change or direct the shop to change.

Thank you.