Mark, yes the first thing is to check the speaker wiring(re do it even if it looks good)for a possible loose wire in the center or surrounds causing a short.

If the external wiring connections are okay, it's possible that one of the center or surround speakers has developed an internal short. Disconnect one of those speakers at a time to see if operation is normal when it's not connected.

-----------------------------------

Enjoy the music, not the equipment.