Other problem I came up with was running guiminer from source. Everytime it found a share it gave "2011-05-27 xx:xx:x: Listener for "username": 27/05/2011 xx:xx:xx, warning: job finished, miner is idle" and hashes came down for a small time.
Never tried to run it from source. Did other pools (maybe that closer to you) behave in same way ?