Try `nslookup -vc [hostname]` after setting your name servers to the Level3 IP. Just a wild guess. I can't remember off the top of my head if nslookup or dig,

Try `nslookup -vc [hostname]` after setting your name servers to the Level3 IP. Just a wild guess. I can't remember off the top of my head if nslookup or dig, when specifying an alternate server, use TCP or UDP. But, if it uses TCP for those, then it's possible that your ISP is blocking UDP DNS traffic that's not destined for their DNS servers. It wouldn't be the first time.