That's right! Everything is pointing to the problem being around the ratio board/switch.
I've slightly lost track of who's had which issues over the last little while, but am I right in thinking that your problems went away when you touched T7? That transistor is right next to that 6-pin header on the main PCB, so maybe something mechanical is at play, a broken trace or somesuch.
Does the problem go away (with the ratio board connected) if you put a small amount of pressure on the connector housing? Is there good continuity from the connector pins to the PCB pads, and to the other ends of the traces that lead from the connector?