Slider, welcome. The first thing is to suspect the wiring, especially when this happens at moderate volume and after a short period of time. When you say that you "checked" the wires, did you actually disconnect and very carefully reconnect the wires even if they looked okay? If not, I'd suggest trying it. You should if at all possible use the 8 ohm impedance setting, so that the maximum power that the receiver is capable of isn't reduced by the voltage-lowering 4 ohm setting. If there's still a problem you can also try running the speakers one at a time to see if a particular one seems to be involved.
