Mine does this on occasion as well. It seems to get "stuck" on whatever channel you last viewed and is for some reason unable to change channels. Sometimes I get black screens with 'x' floating around (which I suspect is the screensaver on my Digital Cable box), other times it just records whatever is on the channel that it is stuck on.
I've had lots of reboot issues with mine as well. It seems that I get the right shows recorded at the right time about 75% of the time, so I've lived with it. I still have a couple months on the warranty and am curious about the posts I've read indicating Philips is offering a replacement model. The lack of the Guide feature really limits the convienience of recording TV shows however.