I used to have an '86 300E that would do the same thing on trips. When I would notice the air output starting to warm, I would simply press the EC switch and let the ice melt. The air output would stay quite cool for a good while because of the ice. When the ice melted, I would turn the AC back on. Seems like I had to do it every 40 minutes or so. It would take about ten minutes to melt while driving.

One way to tell if it's freezing at the evaporator...park the car when you think it's frozen. If tons of water leaks out under the car, you know it was ice melting. That's what mine did.

Never did fix it, btw. Just learned to work around it.

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2012 E350
2006 Callaway SC560