issues with oil pressure

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yesterday my truck decided to start dropping oil pressure when i come to a stop and I was wondering what it could be? now let start with it's a 95 ram 1500 3.9l v6 OK from what i can tell when its cold its just fine! after it gets warm everytime i come to a stop the oil pressure drops and my oil light comes on and Before i replace the oil pump i want to know if that's the problem or if its the oil pressure sending unit please help!