I think there are some web servers (there are many) that are down. The load-balancer sometimes sends you to an 'up' web server and sometimes to a 'down' web server. Bit worrying.
[EDIT] Based on my api ticker - it's failing about 25% of the requests - so I reckon 1/4 of the internet facing web servers are not responding.
[EDIT2] I'm still with Slush tho - and mining is unaffected by the internet facing web servers - loyalty counts - have faith
The LB is supposed to detect that and remove it from the farm. Seems that there's no probe activated in the LB.