Vchimp, was this over-the-air or on cable?
In this brave new world of 16:9 digital TV, stations and networks can send a little piece of data called "automatic format descriptor" or "AFD" along with their video. It tells display devices and distribution systems downstream how to convert a 16:9 image for display on a legacy 4:3 screen. If you're watching standard-def 4:3 cable, and it's configured properly at the cable company's end, AFD can tell the cable company to send along some programming letterboxed, while center-cutting other programming for fullscreen 4:3 display. And it can do it dynamically, so you can go from a live sports broadcast that gets letterboxed to an ad that's center-cut and right back to letterboxing.
When you see the picture jump quickly from "squeezed" to "center-cut," as you did, you're probably seeing AFD being triggered just a little late.
It's not just cable companies that can do this. So can your over-the-air converter box, if you have it on the "Set by Program" setting.
It's my experience that NBC and Fox are the most aggressive users of AFD, especially Fox, since the network no longer tries to make any of its programming 4:3 safe for legacy center-cut viewers.
(And having said that, may I suggest that in a world that is increasingly 16:9, you're only going to get increasingly frustrated trying to make it all fit a 4:3 display. 16:9 TVs have become dirt-cheap at the low end, and most of them come with enough inputs that you can feed one new TV with inputs from all of your various TiVos and VCRs and whatever else you're using these days.)