I've detected problems in bitcoind 0.8.1 where high number of transactions in memory pool caused 100% CPU load on the pool server. Because of this, submitting of new blocks were delayed for tens of seconds and caused high invalid rate.

Sipa (the bitcoin developer) implemented fix for this problem (https://github.com/bitcoin/bitcoin/pull/2677) which I just tested and implemented to the pool server. I'm watching the pool closely, but the CPU load dropped from 100% back to 2-3%, so everything looks fine so far...