Why is my central air / furnace freezing / icing up?

The part that is icing over is the pipe that goes from the furnace outside to the central air unit. I had it checked by a professional and I was told that my pressures & freon levels are fine. My filter was changed a week ago. However, it was rather dirty when it was removed. The professional told me that he didn't know what was wrong. He said my central air unit is rather old. Therefore, I need to buy a new central air unit. My furnace is less than a year old and I am afraid that if it continues to freeze, it is going to hurt my furnace. I turned my furnace off with only the fan running for over 24 hours. It appeared to be completely thawed out. It iced back up once I turned on the air conditioning again. What should I do? Is it possible that the central air unit is fine (since it is maintaining the correct pressure and freon levels)? Is it possible that the pipe going from my furnace to my central air unit just needs replaced?