[Readily Visible]

When I put the top and bottom back on the unit and plugged in all the
various RCA plugs and speaker wires and 300 ohm antenna, the same
problem reasserted itself, the left channel bugged out.

Then I pulled the top and bottom and set the unit up on its side and it
has been playing all day without a hitch.

So I am thinking the problem is either heat related or position related.

As far as position related goes, this time the unit is standing up on
the opposite side as before, so I think the problem is heat related. As
long as the top and bottom are off, the left channel does not go out.
That makes a problem because as long as the problem does not assert
itself with the top and bottom off I cannot poke and prod and measure.

Got it!

I will simply leave the top and bottom off and put the unit on its side
and live with it.