Sudden loss of volume from turntable

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.

This same problem just happened to me and I tracked it down to a faulty IC cable (DIN to RCA).
If you have a 'Mono' switch on your phono-stage.......that showed up the problem in my case more easily?
Good luck.

Thanks I will try those suggestions out, very much appreciated.
Can it be the cartridge?

Could be the cartridge or the wires or clips........but it would be unusual to then affect BOTH channels?

I have a similar problem (total dropout of one channel or the other) and I'm pretty sure it's the ancient wires from the TT to the phono pre-amp. Will be replacing as soon as I get the wherewithal to do it (some soldering required on mine).