To me, this issue sounds like you have a short with your speaker cable. Tapping the unit moves it enough to stop the short from sending the unit into protection, which is why it clicks off. I would try reconnecting all of your speaker connections and then seeing if you have the same problem. Even a little strand of wire touching the wrong connection can cause a short.

I know this is not the same solution as above but you may want to try it first as to rule out all options before sending it back. Good luck!