I'm having the same problems on all of my domains hosted with DH (and have noticed it with friends DH hosting as well). From what I can tell, it appears to be DH DNS servers timing out. I've been looking into it all morning since it's been one of my deciding factors of leaving DH to go with someone more reilable.
I've been running the DNS Timing script that DNSStuff (http://www.dnsstuff.com) has and DH's DNS servers have been timing out intermittently and causing large delays in resolving of my domain names.
I put in a support ticket this morning, we'll see what comes of it.
Other than this small issue, DH has been absolutely great!