Recently my central air system has been "freezing up"...that is, it'll cool the house to a certain temp (around 75F), then where the big, insulated line and small diameter line come into the unit on top of the furnace from the outside compressor will start frosting and icing up, and eventually the air stops flowing through the whole system. I've changed the filter, cleaned out the drain line as best I could, and even installed a new thermostat.

Any ideas? The system is 16 years old, but i sure don't want to have to replace it now. Any suggestions would be welcome

Cheers,
Tom

If the filter is clean then it sounds like you have a refrigerant leak.
Chances are pretty good if you have a leak and call a refrigeration mechanic he will just fill up your system without actually looking for the leak, if it's the first time you've had to service the unit in 16 years you're lucky/unlucky. Lucky that it lasted so long, but unlucky that you may have a slow leak.
If you're handy you can do the following. Make sure the unit is powered off before starting, shut off the breaker or remove the service plug. Have a look at all the lines that are easy to see, if the copper lines seem to be oily, you probably have a leak there. To further test a few drops of dish soap in a cup of water and lather the pipes, you will see bubbles where the leak is, much in the same way an auto mechanic looks for a leak in a tire.
Last try bribing an HVAC guy to lend you his leak detector.. I prefer Baileys

Just to add to what has already been stated, another cause for freeze up is not enough airflow across the evaporator coil (the one inside the furnace). Checking the filter has already been mentioned, but if you have a unit that has a belt driven blower, the belt could be loose enough that the proper amount of airflow is not being acheived.....and the unit will freeze up also. Sometimes people will also close off vents in rooms that they don't use and this can also cause a problem with airflow across the evaporator unit and cause freeze up at the coil. If you have a direct drive squirrel cage blower instead of the belt drive, check to make sure that the motor is actually turning at the speed that it should, i.e. not binding up at the bearings and causing it to run hotter and slightly slower. Also make sure that the blades on the squirrel cage are not clogged up with a lot of dirt, lint, whatever and impeding the airflow across the evap. coil. It may very well be a loss of refrigerant, but you might as well check the simpler things first and maybe save yourself some money. Hope this helps.

I would be willing to bet it has to do with a restriction of airflow across the A-coil (Evaporator) somehow. Like the last post said, check to be sure the fan is working properly, check to see if the blades are dirty, check rooms for vents that are closed (Very common and often overlooked).

Some other causes that you will likely need a technician to troubleshoot:

If your discharge and suction pressures are low, with a low compressor amp draw, low subcooling, high superheat, and low temperature splits accross your evaporator and condenser coils, you probably have a leak.

If your suction pressure is low enough to cause the evaporator to freeze up, but you have high subooling and high superheat, either your metering device is restricted, your drier is restricted, or your liquid line is restricted.