I spent about five hours yesterday taking apart and cleaning the a/c condenser. It didn't help.
When I arrived on scene, the a/c was working fine. Cold air was blowing out of the vents. The refrigerant drier cannister happens to be located on the small line outside the evaporator. This line was very slightly warm to the touch. The larger copper line going into the evaporator was cold to the touch. The outside of the plenum was also cold (not freezing).
After about 30 minutes, the a/c unit's fan turned off, the furnace fan remained on, and the evaporator line outside the plenum, and the plenum, got warmer.
Time to clean the condenser. I turned off the thermostat, pilled the 40 amp fuse at the a/c unit, and disassembled the side panels, the fan, and any wiring as necessary. The compressor had an orange vinyl insulated sort of pull down 'hat' over it. I took that off, too, to get the compressor specs, if needed, and I put my hand on top of the compressor. It was hot enough that I couldn't keep my hand on it for more than a second or so, but not hot enough to cause immediate burns.
There were lots of leaves in the drip pan, but not much came out of the condenser. I first vacuumed both sides of the entire consenser, and there was not much debris at all. I then wetted and sprayed foam on the outside, and rinsed from both the outside and inside, until all runoff water was clear. Actually, I was surprised at how little dust/dirt there was. The only thing that was really dirty was the temp sensing unit that sits on the small copper line, right where it enters the compressor. I removed it and cleaned it off good.
I got it put back together, turned it on. The outside temp was now 100 degrees. The indoor temp was 77, and I called for 66 at the thermostat. Cool air started coming out, measured at 64 degrees (I guess the attic ductwork was really hot) and it ran fine for about 40 minutes. Then, warm air came out. The a/c fan stopped. I hand checked hand temps at the plenum, and it was warming up again.
I'm calling another company out for a another look before I replace anyting major.