91' Cadillac 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, the repair shop spent three hours on it.     Road tested it and duplicated the problem.     Checked the ignition and fuel system.     Fuel pressure was within spec.     Replaced the rotor in the distributor.     Cold cranking amps tested 679Amp just a little below the 770Amp rating.     Trouble codes indicated Camshaft Position Sensor.     Cleaned the connection for the Camshaft Position Sensor and retested.     Note a loose part in the catalytic converter.
But, when I drove it there was really no change in the symptoms. I did discover something new on my next 300 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. During the last 200 miles or so it happens all the time even after car has been running for over an hour. So now it will be easier to duplicate.

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