What worries me is that up and down of brightness.
I had been writing for several hours and not experienced it.
Then I come to this website, and within ten seconds I have had three up-down movements. Not random. Always the same two levels.
It could be that the display is adapting for the huge white areas of the post windows, because after it did this a few times brightness remained stable.
A bit like what the display does when you power up the computer.
This is when you start typing or just hitting the keyboard, right?
You might just have a USB connection issue if at any point you are utilizing that type of connection during and after profiling. I'm not familiar with how your display setup is but just as it has been suggested, test by hooking up another monitor (cheapo or old used one) and don't even calibrate it or hook up any device through the USB or any other connection and see if you get this increase/decrease in brightness associated with keyboard activity.
If it doesn't do the brightness fluctuation, then it may be the internal electronics of the NEC display at issue or near or at failure. If it does with the test display, then it's most likely with your internal electronics of your device connection pathway be it USB, Firewire or whatever is used during NEC connects and profiling routines.