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Have you tried switching things around, such as the phono stage into the same line input used for your CD player? It could be a number of things, but you will probably have to isolate it. Can you bypass the SUT into the phono stage even though the sound output will be low? If the volume drop doesn't occur, then it is likely the SUT. And so on. Maybe you could borrow some equipment to try and see what happens when you switch out one component at a time. It sounds to me as if some electrical component is heating up after a time and then cutting out.