even iso 200 its too noisey, i think the unecesary dynamic range is increasing the noise. you should try just using the -1 and +1. and maybe 1 more if really needed then noise ninja i think itd look a lot smoother.
Actually it's the problem with the 18-135 - I've noticed serious distortions (dark or discolored blots) when I put the focal length around 80mm and above, which don't exist when I use shorter focal length.
For sure the lens wasn't designed with the IR in mind and that's the reason.
My D80 is still unconverted - I may do it, but then I'd need another D80 for the underwater photography as my housing works with the D80 only (cheaper to probably get D40 and convert it).