Ted's route to looking at ignition switch is good, and likely to at least see where the missing volts all leak out
The ignition rotary switches on these as prime suspect can have that trait, just stopping with no prior warning.
The contacts on the wires going into the ignition key mech are on a plate that pushes up inside rear of barrel construction, then to locate with little "tangs" that hold the plate from coming back out.
The contact is made by the key rotation of another internal plate with little copper bridges to connect the wire terminals in correct sequence to power the circuits.
If the bridges or contact corrode, then power is not transmitted to where it should go. There's little steel coil springs pushing the bridges into competent contact too, if these rust then there's no pressure to hold them in secure contact.
If the volts go in there and don't come out again, spray some de moisturiser (wd40 or similar)in the back and rotate the key a few times. It may help it pickup the action without dismantling being required.
