As strange as it may sound, I have a similar problem with my cell phone and
my laptop. I tried to trouble-shoot and isolate the problem(s). Here's what
I found: bending the printed circuits a bit switches the devices from
working to non-working modes, but wiggling any connectors or wires has no
effect. So I'm assuming the problems are with the printed circuits
(mainboard in the laptop).

I can not see any bad contacts or other problems with the naked eye. What is
the most likely cause of such twilight (between working and broken)
symptoms in printed circuits?