Missing event data

Hi everyone !

I am trying to create 3d-shooter-like controls for an OpenGL
application. It works fine so far, but everytime I hold down one or two
keys on the keyboard and simultaneously move the mouse, the event data
of the mouse motion events doesn’t reflect the real movement I did. It
seems there’s some missing event data.
I wrote the current movement per second to stdout and moved the mouse.
Every few frames there was an unexplained drop in movement rate to
almost zero.
It doesn’t matter if I catch the mouse motion events or use
When I don’t press any key on the keyboard it works perfectly. It also
works fine, even when I press keys, as soon as I remove SDL_ShowCursor(
SDL_DISABLE ) from my code.

Any ideas?