The most common cause: A unit that is nearly 20 years
old and has never been cleaned. Dirty fins=reduced air
flow=icing. Google should help you locate many references
to sites telling how to clean the unit. It is extremely easy.
Second most common cause: Low refrigerant due to a leak.
This is often a cheap and easy problem to fix IF you have the
equipment and knowledge to do the repair. However, paying
a pro to fix it is going to be a waste of money. You certainly
own an R-12 unit, and the retail cost of having the Freon-12
installed is a great down payment on a new unit. Add in the
service call/bench charge, labor charge, piercing valve(s), and
brazing the leak and you have a lot of money spent on fixing
a unit that is 2 decades old. Plus, you are going to owe the
tech money even if the repair is more than you decide to spend.
You may have found your excuse to buy a new unit. They
aren't really all that expensive.