tsuami

Recently purchased a '99 C280 sport with 155k miles. Just replaced the entire AC system- compressor, condenser, dryer, expansion valve, hoses, etc. Has worked fine for weeks until yesterday while my wife was driving alone. It was a fairly hot day, maybe 80 degrees with some humidity, and she was running the climate control at 70 degrees with the windows up. After 45 minutes or so of street driving the system suddenly started acting erratic- the passenger dash vent started blowing hot air, while all the others seemed to stay cold- add that the top of the dash became very hot too. She also noticed the engine temp sensor rising, from the usual halfway mark to closer to the 3/4 mark. Shutting off the AC and rolling down the windows brought the engine heat back to the halfway mark.
In the last few days I have not been able to duplicate the problem with short distance street driving. AC seems to blow cold. What gives??