[Revved]

I've got something similar happening with a vehicle in the shop right now with a Vintage Air System. Driving on surface streets with stop and go traffic there is no issue, but extended highway driving (45min+) it starts getting condensation dripping out of the evap case inside the vehicle. I've already verified the evaporator case drain is clear and I get normal condensation drip with normal driving. My guess at this point is that the evaporator is icing up and blocking the evaporator case drain. The car is apart right now for other work so I haven't been able to test my theory yet by playing with the blend and adding some heat into the case to see if it keep sit defrosted.

Try adding a little heat on your blend to see if it helps. These systems run a heater bypass valve to keep hot coolant out of the evaporator case when on full cold so the A/C blows colder...but it could be getting too cold.

If I remember my A/C theory propoperly an overcharged system could also cause an evaporator to ice up. Most of my experience before the pro-touring work was with fixed orifice tube systems rather than expansion valves as all of these aftermarket systems seem to be.