I examined the connections at each of the speakers and at the module at the base. The connections at the speakers seemed secure. It was harder to tell at the base since they are soldered on, but they seemed ok. With the speaker off the wall I was able to replicate the brief fix by tapping, the mid ranges come on but then drop off after 10 seconds or so. When the mid ranges are working, they sound fine.

I'd appreciate any further suggestions. Thanks once more.