This may be a no brainer, but make sure that your recording an actual HD broadcast on a true HD channel vs the SD channel.
That's a really good point.
If your DVR is like most DVR's, you ought to be able to pause & rewind live TV. Right?
So the next time you're watching an HD show that you know is in HD, hit pause, rewind it a bit, then hit play. If the picture quality is the same as what you were just watching, then your receiver is capable of recording HD content. Sounds like this is the case, as you say your Superbowl replays looked great.
I'd bet your problem is exactly as oldschool says, and you're accidentally recording the SD channel rather than the HD one.
