A/C Problem

I have a 1999 Dodge Turbo Diesel, If I turn the vents on for outside air, my A/C appears to freeze up. I can turn the A/C on and it will blow warm air. If I shut the system off and turn it back on it will start to work after it thaws out. This seems to work for about 20-30 minutes and then appears to freeze up again. After turning system off and on once again it will work for 20-30 minutes again. This cycle will continue indefinetely. This started happening after I had my A/C system serviced at the local Dodge dealership. Any help would be greatly appreciated.
Tater202

You had better take it back before long.