When are you noticing the change in volume? Is it between the show and the commercial? When you change channels? Or is it randomly raising the volume in the middle of a show?
TV broadcasters allow commercials to use a layering of their audio that creates an overall louder effect than the TV shows themselves. The commercial is not louder in the legal sense (there are legal limits on how loud any broadcast can be), but the layering of the audio allows the sound to penetrate further and cut through background noise in a way that the TV show does not use. The effect to the casual listener is a jump in volume, in actual fact the sound is simply cutting through the background noise in a much more efficient manner. But it still sounds louder.
Different broadcasters set their maximum volume differently from each other. FOX for example seems to set their overall volume at the legal limit, so that when you are switching channels and browse past a FOX station anything they are broadcasting jumps out of the screen at you demanding your attention.
This may account for some or all of the volume changes you are hearing.
It could also be that your TV has a Dynamic Audio/Volume or Dialog Enhancer feature that is trying to level the audio, and not doing a very good job. A Dialog enhancer for example is designed for hearing impaired people. It focuses on the frequency of the human voice, and everytime the TV broadcast ventures into that frequency range the Enhancer momentarily cranks the volume. Try turning any audio enhancers off, and see if that helps.
If the above doesn't help, and you are still hearing dramatic volume changes when say, watching a movie from DVD or Blu-ray then I would say your TV has a problem. Call the manufacturer.
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