2004 Excursion/Powerstroke/ On Warm days, ac loses power. I turn the key on and there is no lights on the AC. Then, it comes back on and works again. Sometimes after couple of hours. I am assuming there is an electirc relay or something that is defective and in warm weather, acts up. Does not do it everytime in warm weather. Any help would be greatly appreciated.

Could be the relay, the controller, the temperature sensor in the coolant system, the compressor, the R-134 level in the system, etc. What is SOUNDS like is that you may have a bad ignition switch or trouble with your harness. At any rate, the intermittent nature of it (I'm assuming that when you say "it comes back on and works again" that it works normally during those times) suggests a likely electrical issue.