Regarding fuel shutoff solenoids, many times it's the wiring that's the culprit. As everything ages wiring is one of the primary points of failure: insulation breaks down, wires start breaking etc.. Good troubleshooting practice is, as has been mentioned, to bypass the stock power wire to the solenoid. If you've got a good 12v to the solenoid then any issues are going to be with the solenoid (or something else- also quite possibly wire-related [IP wire harness]). The clicking sounds are pretty telling: there shouldn't be any unless the operator has switched the key.