Boy, it's sure sounding like MPEG artifacts, but still hard to tell. A picture would be nice...
Here's a series of questions that might clear it up instantly:
1. Does the checkered static "move" at all?
2. Does the checkered static appear on DVD movies as well?
3. If the answer to both questions 1 and 2 is "yes", then...
4. Press PAUSE on the DVD player when you see it. Does the static freeze and stop moving?
5. If yes, then you are seeing MPEG artifacts on the DVD, they've been there all along and the only reason you didn't notice them before is because the TV screen was smaller.