starter solenoid question anybody

[c10 custom]

hi guys i have a 1979 chev truck and i have a power probe and the small wire on the starter solenoid i have no positive 12 volt power at the wire when the key is on i have a negative power through my power probe and when i turn the key to run the negative power goes out on the power probe wondering if it could be the ignition switch i have headlights and the gauges do flicker when the key is turned on any help would be great