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Do you have a volt meter?
Are you able to put the meter probes in the splices between the supply wire and the fixture wire, and watch the readout on the meter as the brightness of the light fluctuates? With this test, we would be checking to see if the actual voltage going into the fixture is fluctuating.
My apologies for the delay. I'm cleaning up from dinner. Yes, if you can monitor the voltage at the switch, while watching the brightness of the fixture, that would be beneficial as well.
Sounds good. Just reply when ready.
Does the halogen fixture use any type of ballast, or transformer? Some do, and some don't. At that wattage, most wouldn't.
I missed your earlier question about why the voltage would fluctuate. If it were fluctuating, there are a few things that would cause it.
It could be a problem with the transformer that feeds your house. In that case, all of the power in the entire house would be fluctuating, on at least one leg (there are 2 legs of power).
Another possible cause is a loose connection. When a wire is loose at a termination or a splice, it can cause the voltage "downstream" from that location to fluctuate.
Those are all good steps. It sounds like you've been doing this type of work for a while.