As mentioned about the first possibility is that one of the speaker wires or speakers is shorting. The overload is the receiver going into protection.
Disconnect one speaker at a time from the back of the receiver. When the problem goes away that speaker cable or speaker is the problem.
The other possibility is that the Dolby processing is being overloaded by too much input signal. When you switch to stereo the problem goes away because you are no longer using that processing. This is normal. You need to turn down the output of the source. If it has it's own volume control turn it down. You can turn the receiver volume up to compensate. They do make inline audio attenuators that can be connected to lower the volume of your source if needed.