Like Chick said, there are a host of possible problems. One quick test you can do is try to check the compressor operation when it is NOT working (blowing cold). You indicated that when it works, it works well. I am thinking that it may be a clutch coil that gets warm, and then goes open circuit. So, when it stops blowing cold, check to see if the front clutch plate is turning. Let us know.

Now, getting a coil for that may also be a little costly, but it will provide another answer.

Hope this helps!
Peter

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