I see this flickering as well but it only seems to happen around dusk in the ourdoors and in buildings with sporadic lighting. Has anyone found a reason for this or a fix for it?. Other than disabling Crossfire...
It could have something to do with the shaders being used? When I was messing around with the 3.0 shaders I ran into a problem with flickering.
This is off the Fallout 3 tweak guides. (Hes got such a great reference manual.)
http://www.tweakguides.com/Fallout3_9.html
"bAllow30Shaders=0 - If set to 1, this option will allow (but not force) the use of Shader Model 3.0 on graphics cards which support it, namely Nvidia GeForce 6600 or newer, or ATI X1000 series or newer. This can potentially improve performance when using HDR rendering for example. However to force the actual use of 3.0 Shaders after making the above change, you will also need to check your RenderInfo.txt file, found under the same directory as your .ini files, and near the bottom the file check which shader package it uses (e.g. Shader Package : 13). Then go to the \\Program Files\Bethesda Softworks\Fallout 3\Data\Shaders\ folder and rename that particular package to something else (e.g. rename shaderpackage013.sdp to shaderpackage013._bak. Now copy shaderpackage019.sdp and paste it back into the same directory, and rename this new copy to the package name your card uses (e.g. rename it to shaderpackage013.sdp in this example). This will supposedly force Fallout 3 to use the Shader Model 3.0 shaders in the game. In practice this doesn't appear to noticeably alter image quality or performance, so at best it might be used for troubleshooting purposes (Fallout.ini)."
When I was messing around with it, one of the files I tried made all the water disappear, one gave me weird flashing stuff, and one worked just fine. I really don't remember which is which. But it might be something for you to check out?
Also with the DLL fix it would change what it sees your graphics card as. Then it might use a different shader then what your card would use. IDK, just another guess... You can check C:\Users\(username)\Documents\My Games\FalloutNV\RendererInfo.txt with the DLL in, then take it out and load launcher so it can detect, then check the shader package again then reinsert dll and do the fix above. Hope this helps.