I don't know what's going on with this. I've got "sneering punk" as my karma level in most NCR areas after killing what's her face in Crimson Caravan for Cass. I've had that for a while, though, and since then I've done quests for the NCR including getting the Boomers on their side. However, during the process of delivering the news about the Boomers to the Ambassador, I had NCR MP's randomly attack me on the 2nd and 3rd areas of the strip. I reloaded and snuck past them to deliver the news to the ambassador, who told me there would be more work for me, before running away as the entire building went hostile on me. Every time I go near NCR troops, whether I'm wearing one of their uniforms or not, they go hostile. What gives?