I have no intimate familiarity with the big bodied cars, but it sounds like jcciem might be on the money here. The reason I think this might be true is that you are getting condensation on the low side line meaning that the refrigeration portion of the system is cooling.

Another possibility COULD be a fresh air door open bringing in hot outside air, bringing up the vent temp.

These observations are based purely on theory with no intimate knowledge of your particular ACC system.