backfires with headlights on

My FWD, 91' Fleetwood Cadillac, w/ 4.9L V8, started to run rough about five minutes after starting from cold and ran rough for the next 15 minutes. I would punch the accelerator petal to alleviate the symptom
for a short period of time while continuing to drive. This started six weeks after I bought it in May 2003 and the first time I had a tank full of 10% ethanol. The car has been doing that ever since (2 years). However, in Dec 2005 it started to run very, very rough (lost power dramatically and backfired). A few times the car's speed went from 60 down to 25 on the freeway and as soon as we pulled on the shoulder for a minute or two we could accelerate again for the rest of the one hour trip. It acts like the vacuum advance stops working temporarily. Typically happens during warm up period (about 5 minutes after starting till 20 minutes after starting).
In early Feb I let my mechanic spend four hours on it. They road tested it and experienced the problem. They checked the ignition and fuel system. Replace the worn two year old rotor in the distributor. Tested the fuel pressure and it was good. Cold cranking amps tested just a smidgen below spec (6 month old, large Wal-Mart battery). Trouble codes indicated Camshaft Position Sensor. They cleaned and secured the connection for the Camshaft Position Sensor and retested. They thought they might have had it licked.
But, when I drove it there was really no change in the symptoms. I did discover something new on my next 130 mile trip. When it starts to run rough I can turn off the headlights and it smoothes out ASAP. If I turn on the headlights it runs rough instantly (no delay whatsoever) The heater motor blower and the brake lights have a similar impact, but not as pronounced. And, this happens now even after car has been running for hours.
Any suggestions?
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A voltage related problem.
Suggest you check these: Alternator output voltage Battery connections AND the other ends of those cables Engine ground connection
There may also be some kind of voltage / polarity protection device (or relay) that's designed to protect the car's electronics during battery jumps etc.; if so equipped, that ought to be checked.
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