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Blowers gradually die after driving 20 miles

After I drive my car for about 20 miles on the freeway the air blowers gradually die off. You can still hear the fan working, and the difference in speed when you change the switch from ‘high’ to ‘low’ and from un-circulated to re-circulated air. The AC compressor and radiator were just replaced as was one of the valves in the AC circulation system. After turning the car off and letting it sit for a bit the blowers work perfectly again. One thing I noticed is that if I stop driving and turn off my car before they stop working I hear a hissing coming from the engine, but if I drive until they stop working and then stop and turn the car off there is no hissing. It strikes me as either a pressure issue or possible a computer issue. The powertrain control module and constant control relay module were also just replaced.

Perhaps the AC evaporator is freezing. That would stop any airflow coming from the vents, even though the fan is still working.

The last time this happened I could feel water dripping on my feet while I drove–would that be another symptom?

Yes, and you only have one blower.

I like the A/C freezing idea as well. If this car has vacuum operated shut-offs in the vent system (which I am pretty sure it does), you may want to check and make sure that you don’t have a vacuum leak that is allowing the vents to slowly close-up as you drive. If that happens, no air would pass through the A/C, causing it to freeze up.