Thanks for all of your ideas and comments.
To answer some of your questions:
Yes, if I put black electrical tape over the problem PC it instantly stays on and does not flicker. Just a reminder that when I first hook up the fixture it does not flicker right away. But then 5-20 minutes later it starts to flicker and does not stop.
If I put black electrical tape over a properly working PC in the daylight it comes on within a few seconds and stays on. It then goes off within a few second once I remove the tape. It responds quickly but not as quickly as the flashing that I am experiencing with the problem PC.
The fixture is designed for a CFL and all of the other identical 25 fixtures on the property have CFLs and have never had the problem.
When I checked the voltage with my digital multimeter I remember it being a constant 120 or thereabouts. I will double this reading next time I am there.
The way the building wiring is set up is it a long single level building and this fixture is at the apartment that is the farthest from the mast head.
After the power comes down from the mast it goes into a service room with the meters. There is a main breaker for this apartment on our side of the meter and then the line travels about 50’ underground and then about 150’ through the attic past 5 apartments to this apartment on the end.
This apartment has its own panel in its apartment that controls the fixture.
The neighbor apartment (one apartment closer to the meters) is an identical apartment with an identical PC fixture that has always worked correctly.
Although, I don’t think we are leaning towards it being a problem with the fixture. Here is a link to the fixture so that you can visualize it.
http://www.amazon.com/Nuvo-Lighting-60-3946-Photocell/dp/B0020S4VS2Note the PC eye points down behind the bracket arm.
I have not contacted the manufacturer for any known problems.
I will be going back to the property in the next week or so and I will take another swing at resolving this. I will check to make sure all of the connections are tight and check for fluctuating hot, neutral or ground. Anything else you suggest I check?
Thanks,
Todd