You took 50 feet of 12/2 to this motion light and it is tapped off a junction where an attic switch is?
You took 50 feet of 12/2 to this motion light and it is tapped off a junction where an attic switch is? Why did you change the attic wiring? From 12/2 to 12/3 and then wire this in? I am trying to figure out if you were compensating for amps or what. Two things, one get a volt meter and go measure the voltage at the 50 foot run. Flip the attic switch while you are measuring [have someone do it] and see if it changes. You either have some kind of volt drop or feedback through the attic switch or something is going on. Can you temp wire just and old regular light in the motion detectors spot, and flip the attic switch to see what happens? motion detectors are picky Let me know all the above.