1970 Mach 1 Headlight Problem

two possibilities here;

1: the headlight switch has gone bad again

2: the dimmer switch has gone bad.

i am betting on the dimmer switch, i have had to replace a couple for the very problem you are having.

04mustang70,
Relays for your headlights are one of the best upgrades you can make to help avoid stressing your 40+ year old wiring! A "key on" relay fed accessory fuse panel is a good addition also and easier to do at the same time as the headlight realays, or at least allow for it in laying out your headlight relays. Here's a good write up, for Ford trucks, but still the same set up:

http://www.ford-trucks.com/article/...o/article/Whiter_Whites_Brighter_Brights.html

Personally, I would ignore/eliminate the "Forced Low Beam" and "High Beam Flash" portions of this set up. It is really easy to do and can be done without cutting any of your OEM wiring. Simply extend the 2 hot wires (low beam & high beam) from either one of your three prong headlight plugs (the third wire is ground) to the relays for control. and make a new harness to go from the relays to the lights.
Post back with any questions and/or to let us know how you make out.
HTH,
Gene

The headlight switch has an internal breaker. It's getting hot and breaking the connection. When it cools off, your headlights come back on. If you're running aftermarket headlights, they're probably drawing more current than stock, and flipping the breaker. The permanent fix is a relay kit.