That's called interlacing. Some shows are interlaced and others are deinterlaced. From my experience, most sitcoms are interlaced, and most movies and dramas are deinterlaced. This is not always the case though (such as with Dexter).
The easiest way to fix it is to set the deinterlace option to "fast". But, only do this for shows that are interlaced. (Doing on deinterlaced video is pointless and will slightly degrade the quality of the show).
The decomb feature is supposed to be selective and allow you to have the option set to "Default" and it will leave deinterlaced video alone but get rid of the lines on interlaced video, but I haven't had luck with it, and don't know how to tweak the settings. Everytime I use it, the problem is usually fixed with obvious motions, say... someone waving, but anytime someone lifts a glass to their mouth, or talks, I can see motion blur on their hand or their mouth.
