Did it switch?
And DNS Lookup Failures is an error. I can write code to just retry though on that specific error if you are proccesing getworks. If you are submitting work it will currently never change servers although it will mark them as lagging.
Thanks for the clarification and you are correct, it did not switch.
I am not sure are those related or not but here is one more error, where long poll can cause switching:
[11:00:07] RPC request [getwork] submitted to BTC World
[11:00:07] LP triggered from server btcworld
[11:00:07] Reading LP Response failed
[11:00:08] RPC request [getwork] submitted to BTC World
[11:00:09] LP triggered from server btcworld
[11:00:09] Reading LP Response failed
[11:00:12] nofeemining: 2520502
[11:00:12] Server change to ozco, telling client with LP
[11:00:12] LP triggered serving miner
[11:00:13] LP Call ozco.in:8332/LP
[11:00:13] RPC request [getwork] submitted to ozco.in