Popping noice/center turns off

This is my first post (from USA). I have an older Kenwood receiver, to which I've attached a center speaker and a sub-woofer. After it is on for a short while (all speakers are working) I hear a "popping" sound, and then all front speakers turn off (when in 3-stereo mode) and all I hear is a faint humming sound. If I change to just 2-stereo mode, I get sounds from left/right and sub. After the unit is off for a while, when I turn it on, I get 3-stereo mode again. Sometimes, after turning off, I hear a popping noice from the sub-w. I think I have all speakers connected correctly. Any help would be appreciated. Larry