This is an odd suggestion yet I found that the problem seriously reduced when I added a line of code that changed the window's title (I was using it to output information) so your suggestion isn't as odd as it sounds.
It might help, because depending on how fast a game update is handled and depending on precision (and the way fixed framerate is implemented), in theory you might screw accuracy due to rounding issues. Although the new sf::Clock API shouldn't have that problem anymore.
