<Space> keystroke lags behind other keys

Hi,ey Since a while, whenever firefox starts lagging, the space key is registered after other keys are pressed. I.e. when i type something like "this is a test" i get something like "thisisat est"; the space key is taken into account but at some random moment after some other keys are taken into account. What couldbe the cause and what can I do to stop this ? Thanks ! Ted