I often shoot indoors for interviews with my D90, but I often get a sort of strobing with it. Check out the attached picture, to help you see it on a picture, I added red marking, so simply look on the curtain on the right of that.

My best explanation so far was the type of lighting in the room, which is common in most offices, similar that was is in the picture:

Does anybody know what is going on? Similar experience? Solution? Explanation?
Thanks for any input.