So, isn't there a bug when this appears regularly?
Eg. some clients trying to send shares with bad nbits or whatever (I know this looks like nonsens

, well simply shares [edit]of high enouhg difficulty[/edit] that are unacceptable as blocks for some reason), so doing unusable work?
Do very long rounds appear after retargetting at other pools also?
Could be a problem with the pool - from my experience tho' slush is usually on things like a rash.
We do seem to have bad luck after a big diff increase - people panick and switch pools for a few days while the hard core carry on. Then things get back to normal for another 8 or 9 days. Then it starts again with another diff increase.