Long connection times

Any ideas what could be causing this? Seems very consistent that it's about 10sec and every 20 requests.

This one from Gloucester UK, using IE7 (site is in UK)
http://www.webpagetest.org/result/120607_R6_PQR/

And this one from Dulles using Firefox. I've cut the file sizes down since this, but the pattern remains.
http://www.webpagetest.org/result/120607_MS_G5Y/

Thanks,
Ian.