Hi seharper. Welcome to the forum. Can't diagnose this without seeing it or having more information, but it sounds typcal of low charge. The compressor is working if you get cold air at times. The clutch could stop working when it gets warm. It has a coil of wire inside that can develop an intermittent connection that is heat-sensitive. In that case you would replace the clutch, not the compressor.
The first thing to do is watch the clutch operation when it stops cooling. If it is cycling on and off rapidly, say once every few seconds, or if it only runs for a few seconds, then stays off for a long time, perhaps half a minute, suspect the refrigerant level is low. This is to be expected after eight years. A simple recharge will solve the problem. That cycling of the clutch is also proof that it is working.
If the compressor is running for around fifty percent of the time or longer, and typically five seconds at a time or longer but it is not blowing cold air, suspect blockage in the system. It is normal to have frost buildup on the hoses, but there will be a heavy buildup, even ice forming at the point of the restriction. Look at the hoses for signs ice indicating blockage in a hose.
Wednesday, June 30th, 2010 AT 1:15 PM