Yeah, just sounds like one of those badly made discs - but saying that, some drives are pickier than others...sounds like either could be the case if the disc plays sometimes, and sometimes not.
Is there just the one DVD drive in your PC? If so, what's the make?
Is it a DVD you own, or are you trying to watch one that you've rented? If it's rented, who knows how it's been treated by people who have rented it before you.