Jim, it seems like switching makes the big difference. Presumably there are 3 longer wires running a distance to the switch. Might one of them be getting to close to the B+ wires, or something like that.

Although It hardly matters, the switch is for base “boost” I think, but the boost seems to be bypassed while in the ‘IN” position, and in use while in the “out position”.. confusing to me so far. ??