You are wise to switch to the 'fan' mode when the A?C freezes over.
There are a few things that can cause the evaporator coil to freeze:
1. Not enough air flow over the coil to give the refrigerant an exchange of heat. The fan needs to be running on its highest setting, cold air moves slower than warmer air.
2. Too much refrigerant is comming into the evaporator coil. This can be caused by a faulty metering device; which is very rare.
3. The condenser coil (the one outside) is too dirty and needs to be cleaned; this should be done every year.
4. Overcharge or undercharge of refrigerant. Either one can cause icing.
CAUTION; if you call a serviceman and he tells you it needs 'a little gas', ask him where the leak is. If he can't tell you, he doesn't know what he's talking about.
5. If you have had it serviced in the last few years and they added refrigerant to the systme, it is probably overcharged. A dirty condenser coil can make the low side pressure appear to need more refrigerant. This will help temporarily but it will go right back to trouble in a short while. The key is 'CLEAN FIRST, GAS LAST'. Many servicemen don't want to put their hoses on the High side because it gives them frost bite when they take it off; again, they don't know what they are doing. When refrigerant has been added repetedly without cleaning the condenser coil, the high side pressure can go well above 400 psi, which is dangerous.
Let us know how you come out with it, we may be able to help further.