[onewhippedpuppy]

Before calling out a tech for a minimum of $100, do the following.

1. Defrost as described
2. Change your filter. Be careful of filter choice, on my older unit the new high filtration/low dust filters really seem to reduce airflow. I find the cheapo blue fiberglass ones work best for my antique.
3. Buy a bottle of A/C coil cleaner (good hardware store or HVAC house - not Lowes or HD)
4. Clean your a-coil and condenser unit. Process depends on your cleaning product, most require soaking spray on the fins, a few minutes to soak, then rinse with water.

That will insure maximum airflow over the coil. If it's still freezing up then call your A/C guy. As a bonus a clean coil, clean condenser, and new filter will insure that your system is operating at its maximum potential.