You need to check the frame ground connected to the passenger headlamp wiring and also the sheetmetal ground on the drivers side. Loose grounds can cause major issues. Also check the terminals in the headlamp connectors. The get loose and brittle over time. I've had recent issues with them. Also check to see if someone has added a headlamp relay kit somewhere near the lamps. If so could be a sticking relay. Here's a link to the wiring diagrams. Hope that helps.
http://forum.73-87chevytrucks.com/sm...c,11766.0.html