Blinking headlights

[donut]

Vehicle in question is the '86 C-30 in my sig.

When I turn on the headlights, they start intermittent flashing. They'll come on for about a minute, and it seems when they "warm up" they start flashing. low beam, and low-high together. Marker lights and turn signals appear to be fine, the flashing doesn't effect them.

I have changed the light switch, (sight search seemed to indicate this could be a problem). I have cleaned any grounds I could find, and traced wires from the firewall to the last light in the chain and found no breaks, nicks, burn spots etc.
It'll also do it if all 4 headlights are in or just 1, no difference.

Truck has been like this for a few months and now I'm at wits end. On top of it all, I'm not sure which direction to proceed.

Nothing major electrical has been done to the truck just years of neglect.