The air coming out of our downstairs vents is cold, but does not come out very strong. Upstairs vents feel cool, but no air blowing out. We have replaced return air filter, replaced the thermostat, cleaned the condenser coils, and sealed possible duct work leaks at the furnace, as well as checking duct work in the attic. Could there be an issue with the blower motor?

The beginning pipes do freeze up after its been turned on for a while, but there is poor air flow from the beginning, so we do not feel that is the problem.

You say you cleaned the condenser coil, but what you really need to clean is the evaporator coil (the one at the air-handler). At this point, you also should pull the blower assembly and clean the blower wheel vanes. If your furnace is a high efficiency unit (90+) you will also want to clean the secondary heat exchanger coil located directly above the blower assmelby(squirerll cage). So when you have the blower out to clean the wheel , get up in there and clean the secondary heat exchanger (looks like a radiator). Just careful not to bend the fins on any of these components. Doing these things should increase your airflow. If all these areas appear clean, check the capacitor for the blower motor (believe it or not, a bad cap can cuase slow running of motor). If that is good, and all other areas are clean, it may be time for a new motor. Normally motors don't just slow down, they just flat out quit, or at least start making a noticeable noise when they are on there way out. Let me know what you find. Good luck.

We have a sheet metal plenum above the furnace so its not an easily accessible evaporator. We have read some things about blower speed, could that be part of the issue?

If it freezes up, it will block flow as well.

Yep, gaining access to the bottom side of the evap coil can be a real bear. Many times the system refrigerant needs to be reclaimed, the lineset cut at the coil, the plenum opened up (not easy in many cases) the coil removed, and cleaned, then everything needs to be hooked back up correctly, system vacuumed and charged correctly, etc etc. This really may be a job for a pro.

Is this a new system? If so, blower speed could be an issue. However, if this is not a new system, and has worked fine for years, then you have a lack of air flow, or a low charge of refrigerant. You say the airflow is poor even when you first turn the a/c on, and it takes a little time for it to freeze up if its low on refrigerant. Really makes me think you have a plugged evap coil. Remember, once the system has frozen up, it can take many hours for it to thaw when turned off. Let it set a complete day and night, then turn it back on the next day. See if its better now. If not, I sure think you need to take a close look at the coil. I had to clean two coils just today. Probably half a dozen so far this summer. Very common problem with central a/c. Once you go through the process of cleaning the coil, blower wheel, and secondary heat exchanger (if a 90+ unit), or at least once you have received the bill to have it done professionally, you will learn to appreciate the importance of keeping a clean filter year round.

Also, as Strat mentioned, once the coil freezes up, it will stop almost all airflow, but lack of airflow is usually the culprit to freezing.

Final thought. Time to get a pro in there. They can check the charge, airflow, clean any components that need cleaning, and overall do the service your unit needs. Please let us know what is found. Take care.