Verifying the checksum is a good place to start. If the DVD fails, verify the checksum of the downloaded file itself. That's where I pick up errors, mostly. But I have a rather unreliable Internet connection.
Make sure you're burning the DVD as an ISO, not as a data DVD. Different burning applications this this by different terms. I've been confused by some. K3B is pretty clear about it now, I think it even asks if you're sure, if you're doing it wrong.
In the past, burning the CD/DVD at the highest possible speed caused errors; slowing the burner down seemed to increase reliability. Don't know if it's still a problem or not, but it's something to try.
You didn't set the BIOS to *not* boot from the DVD drive for security reasons, and then forgot to set it back? Something I'd do.
Good luck, and let us know the results.