Is it the amp or the wiring?

I'd be grateful for thoughts on a strange coincidence.

I had been struggling with my beloved Luxman L-450, which was refurbished and recapped by Jude Hercules in Brooklyn. It sounded great, but every once in a while the left channel would crap out. If I turned the power off and then back on again, sometime it would come back to life. Also, if I turned the selector to an unused input, then turned the volume way up, that would also sometimes bring the channel back to life.

I took the Luxman back to Jude, but try as he might he could not reproduce the problem.  So I put the Luxman aside and subbed in my Rega Brio.

For a while, the Rega worked great, no problems. And then . . . the left channel started crapping out! Even stranger, I could get it going again by turning the amp off and then back on again. I had been using the Brio for years in a different setup without this problem.

So two amps in the same setup, both having the same problem fixable by the same solution.

That suggest that the problem is not the amps but the setup. So here is the setup: The source is an Auralic Altair and the speakers are Heresy IIIs, all bought new. Also, the speaker wiring is in-wall wiring. It was done by a competent electrician, but I am growing suspicious. Could that be the problem? What are some other areas to troubleshoot?