Just opening up Google Chrome slows my AOCLBF Phoenix 1.5 miner down from 430 MH/s to 383 MH/s ...
Does this happen for anybody else?
It does not do this with IE 9...
Only if that card is used for the display. If the card is what is connected to your monitor, than that is to be expected, doing anything on the machine takes some processing, generally of both the cpu and gpu, some things more than others.