If you have eliminated the phones jack, and if the phones are also cutting out, the jack seems innocent, then check the other things I mentioned elsewhere, in particular that resistor.
Isolate the problem, plug a cord into the effects send jack and connect it to the input of another amp so we can listen to what comes out over there. If that fades away too, then the preamp has the problem, but if that stays strong, then the power amp has it.
Did I suggest connecting a spare cord from effects send to return? Try that.