engine dies after idling for 5 minutes, then starter won't turn over

[1QKSNK]

Just as the subject line says. I have a SPF early model with a 427FESO. Started fine yesterday afternoon, went to pick up my son, cruised around and stopped at a buddies to pick something up. Left the auxilary fan on as I ran inside. Showed my friend the car, and while we were looking at it, the idle dropped and the car shut off. NO POWER AT ALL. when I turned the ignition key, I got no ignition light, nothing. Just a clicking starter. However, the auxilary fan ran. Strange. After trying to jump it off my portable charger (which made the lights come on, but wouldn't turn it over), we tried to jump it off my buddy's car. After a few minutes it worked.

So I'm at a loss... something is obviously leaching the power, but I have no idea how to find out where. Maybe a bad alternator?

Should also mention that I have been running the brights as the regular headlights have been VERY dim. I just replaced the battery in August.

Any suggestions are appreciated!