a/c freezing up after 1/2 hour. Is there anything I can check?

A/C seems to run normal for a while then slowly gets warmer and warmer. Big puddle and even ice chunks under car after 1/2 hour stop. The car is a 2002
XG350 and no service of any kind has ever been done to the A/C (so its not overfilled). The little green light on dash goes on and off as I dial the desired temperature up and down and I can feel the A/C turn on and off, but I can't tell if the A/C is cycling properly when the desired temp is set to a constant 70. Could the compressor be on all the time? Any checks a shade tree mechanic can do?
Thanks
Dan
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Presuming the car was produced prior to 5/15/02, the evaporator thermistor has been updated and its location changed to help prevent evaporator freeze-up. Check www.hmaservice.com for TSB 03-97-001.
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