Our AC works for a while then stops cooling. If we set it for 78 degrees, it will cool to 78 then stop. When it comes on again it will send out lukewarm air that is very humid. To get it to start cooling again we need to turn the thermostat down really low, like to 60, then it becomes too cold in the house. The unit is 3 years old, it's a Carrier 3 ton HVAC unit 12 SEER sold by Sears, and has never really worked right. We have had coolant added and diagnostics done. We were told our ducts were to small and in bad shape so we just had them fixed, but the problem continues. Our house is a 1700 sq. ft. one story built in 1974. We live in the central valley of California. It's very hot and dry here in the summer. Any ideas? Thanks for your help.

That could be any number of problems, is the outside unit running during these "lukewarm" spells? Is the fan on the outside unit running but not the compressor?

Sounds like the thermostat is out of calibration. If you know what you are doing you can jump the thermostat out of the circuit and see if it starts coling right away

OK, thanks for the answers. I will have the thermostat checked out next.