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The consumer CMOS sensors installed in todays digital cameras are active devices. Each pixel is composed of a photosite (the light sensitive bit) co-located with a processing circuit. Consequently, CMOS sensors have characteristic patterns of electronic noise.
This is normal, but typically different for each camera model. Banding is a function of CMOS operation. Some worse than others. To obtain good results from CMOS images, they must be calibrated. In many cases dark and flat frames are adequate for this purpose.
There is nothing wrong with the camera, it's just that we are asking it to do something for which it is not specifically designed - unlike CCDs which are passive devices (please correct me if I am wrong about this) and usually cooled.
I would use in-camera reduction if taking only a few images at a time. It is very convenient, though not necessarily suited to large image sets.
This digital low light imaging thing is hard work at times and there are no shortcuts.
You are stacking lines of electronic noise layer upon layer - the effects are devastating to your data.
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