I've had the same problem with both live channel changing *and* recording over the past month or so. I'll have it set up to tape ABC (Jeopardy) in the evening, then FOX (American Idol), then want the recorder to switch over to, say, "Lost" on Wednesdays. It will successfully record ABC for Jeopardy, then go to American Idol on Fox, but flip out and refuse to change to ABC to take care of "Lost". I've taped the local Fox news 2 weeks running, while the TiVo menu continues to insist that it's really "Lost" (no pun intended).
I have a Series 2 Humax, using Cablevision with a Scientific Atlanta box, and I've lost total TiVo functionality twice over the past four weeks, as something (cable box or TiVo) just "snapped off" in the middle of my watching a DVD (once it ended in blue screen and inability to recognize any signals between the TV, TiVo, or the cable box; once I was able to restart TiVo immediately, another time I had to wait for an hour or more). This results in my a.) being unable to change channels live the next day after shutoff; and b.) the aforementioned taping issues.