Occassional "Waiting for response from" lag

[raymond]

As you can see from the test results, I have a pretty fast loading website.
http://www.webpagetest.org/result/131126_A0_H2A/

However, my assistant administrator randomly experience a problem when trying to load our website (raymond.cc).

He managed to capture this on video. What he did was refreshing the page and at 19 seconds of the video, he gets a "Waiting for response from raymond.cc" that last for a few seconds.
http://screencast-o-matic.com/watch/cIXOXKVRP6

I understand that this problem can be caused by either the user's computer or the server. "IF" this problem is caused by my server, may I know what is the possible cause?

It can't be DNS because our website was previously using DynECT and now on EdgeCast Route. Both are enterprise grade DNS service that cost quite a lot of money every month.

Any ideas is much appreciated. Thank you.