A/C PROB

IM HAVING SAME PROBLEM WITH MY 97 LC,HAD NEW PRESSURE SWITCH INSTALLED AND RECHARGED,STILL THE SAME,SEEMS LIKE IT FREEZES UP BUT DEALER SAID THAT ONLY HAPPENS WHEN LOW ON R-12,NO TELLING WHEN IT WILL HAPPEN BUT LET IT SET FOR A FEW HOURS AND IT WORKS AGAIN.CAN ANYONE HELP?