Sometimes QAM can have problems. I'm not a cable tech, but I've talked to the one that came to troubleshoot my situation, and have heard from others on a local board here in Rochester NY.
In my situation, I would get drop-outs (green pixels, then black, then back) on the ABC channel (1013 on our TW cable). It always seemed worse during Monday Night Football!
The tech explained that the RF frequency that they use to pump this particular QAM channel is the same as the local FOX analog OTA frequency. You can do that, because the shielding of the cable keeps the TW stuff inside, and the OTA stuff outside. But if the shielding isn't pefect, you can have "instrusion". In this case, I had some crap RG56 cable that the home builder had used, along with some questionable splitters. The TW tech replaced cables, splitters, and things were much better. Still not perfect, and this is a recurring problem with other customers.

Another issue that has been driving installers crazy over the past few weeks is the introduction of a few new "local NTSC stations via digital" channels. They used some new QAM channels to carry the local digital (SD), and these are at the top end of their QAM assignments. They're a higher frequency than anything else, so they're getting digital drop-outs on "marginal" cables/splitters that worked before, but can't handle the higher frequencies.

The best option would be to find a local board that deals with reception issues in your area. There are sections on AVSForum for local HD reception, if you don't find anything local. Ours for Rochester NY is http://www.rochesterhdtv.com