I'll try to explain what happened from the point (I think) that it all started.
My reputation with the NCR was never great, but we got along Ok and I was doing their missions. I think my status was usually 'accepted.' Everything was fine until I picked up Cass and did her mission which involved wiping out the Crimson Caravan Company. This upset the NCR (which is fine) and I think my rep with them dropped to 'Merficul Thug.' At this point the NCR started shooting at me on sight. In the process a few more NCR people did get killed (mostly by my companions) which caused the message "NCR infamy gained", but my status with the NCR never dropped below 'Merciful Thug.' It's worth noting at this point that everything I have read suggests that this is a neutral status and should not lead to hostility. I have also never put on any Legion or Powder Ganger clothing/armour. My status with those two has always been "Vilified". During this whole time I wore only neutral clothing. Also, another point of clarity - before this I never stole from, killed or upset the NCR in any way.
At first I thought this must just be how it works, so I did what I could to try to improve my reputation with them again. This was very difficult as I couldn't easily even get near them. Anyway, after a time I realised that if I wore NCR clothing my status changed to "Neutral" and they weren't immediately hostile. This enabled me to get going with the Bitter Springs quests. However, at this point it became apparent that even with my NCR gear on, certain characters were still being immediately hostile towards me. This was most commonly, but not exclusively, named characters.
Example 1: If I attempt to convince Colonel Hsu (I think that's his name) in Camp McCarren to donate troops to Bitter Springs, he and almost everyone in the whole place are completely fine with me and treat me like an ally. The one person in Camp McCarren who is hostile and tries to kill me is the Captain that I need to speak to about leaking NCR information (or something). This makes this quest impossible to complete. It makes no difference if I wait for days. It makes no difference if I complete other quests in between. If I go in with no allies and don't fight back, other NPCs don't even join in. For example, Colonel Hsu will stand and watch as the Captain chases me round and round his office shooting at me. The Colonel's marker on the compass does not turn red. The other guards are similarly disinterested.
Example 2: If I go into Camp Forlorn, the unnamed sniper on a little platform will shoot me on sight if he sees me. In this case, this will prompt everyone else in the camp to also turn on me. If he does not see me, though, I can wander into the camp, talk to everyone and start doing the quest there. Incidentally, before this guard sees me, (this is the case with most of these examples) his marker on the compass is white. When he turns and sees me it turns red.
I could go on, but in summary, almost every NCR controlled location will have one or two or three people who are immediately my enemies. These are mostly (but not always, see above) named characters. Their hostilites sometimes cause all their friends to attack me, but sometimes they just don't care. Aside from the few NCRs getting killed early on, (as I explained above), I reverted back to a save after any of these things happened, so I'm not compounding bad reputation or anything.
I've tried everything, but I know now this is just a really annoying bug. Other solutions I've heard include putting on and taking off again other factions' armour. This doesn't work. Someone suggested completely dismissing companions. This does not work. The only benefit of going in alone is that I don't have someone with me who wants to immediately run in and shoot up the place. I even managed to complete a couple more NCR missions and the game told me that I was now "Accepted" by the NCR. This makes no difference. As soon as I take off my NCR clothing, my status now changes to "Soft-Hearted Devil" and every NCR member instantly turns red and starts attacking me.
Incidentally, as a side note, I have the same problem with the Brotherhood of Steel. In this case, I had already completed the normal BoS quests and I went back to the bunker with Veronica to do her quest. I accidentally, however, walked in there with my NCR gear still on. The guards at the door immediately started shooting so I turned and ran out. Running out, of course, saved the game and I didn't have a recent prior save to go to, so I went outside, put on my power armour, waited a couple of days and went back in. Now, those same guards are my friends again and every other unnamed person in the bunker is my friend. Every named character, however, attacks me. This does not prompt hostility from any of the unnamed characters. Once again, no amount of waiting or doing other things will fix this problem.
Sorry, I know I've ranted on for a long time. If anyone knows of any solution to this, or has heard of this bug, or knows any kind of work around, please please help me. I've gone from really liking this game to being thoroughly annoyed with it.
Thanks in advance,
Paul