'91 deville backfires when headlights are on

my '91 deville backfires and loses power when any power accessories are turned on. (ie: headlights, power windows, wipers, rear defroster)
I work for advance auto parts and have asked many professionals what they thought it could be, and they all are as stumped as I am. The car has new plugs, wires( yes, they are on tight and in correct order), cap, catalytic converter, filters, injectors, fuel pump and regulator. I refuse to believe that there is not a single person out there that has experienced this problem. Pleas help me before I'm forced to junk it.

I would check your charging system with your lights on. A poor ground or power lead from the battery can also cause a voltage drop that might affect your computer and ignition system. Check the leads at the starter too as there are several fusible links there that might not be feeding the ignition enough juice.

check your head light fluid, it's probably low!!!

Sorry, I had too. lol

This problem happened to me years ago with a lincoln town car. As I recall it was either a bad battery or alternator not being able to put out enough juice and the various sensors were not getting the correct voltage and in turn were giving the computer faulty data.