I was having a problem with my Block Erupters that when I had two running, which ever one started second would act weird - it was reporting a really absurdly high hash rate but nothing ever got submitted to the pool.
Turns out that this is the result of my USB hub not having enough power capacity to run more than one device at a time. Once switched to a powered USB hub (and once I remembered to actually plug in the wall-wart instead of still running it off of my system power), both of my Block Erupters started working consistently well.
This is the powered hub I used: D-Link Hi-Speed USB 2.0 7-Port Powered Hub (DUB-H7) (amazon:
http://amzn.com/B00008VFAF?tag=jes5199-20 ) It's the one that is popular in the Raspberry Pi community (they do a clever thing where they plug the Pi into the power-only slot and then run the Pi's USB devices through the data ports).
(newbie forum post because i'm a newb.)