Hi,
I'm using KDE4 4.1.81 with openSUSE 11.1RC1 and during playing a video in SMPlayer or Dragon Player window and playing with other's windows focus (I've focus set to follow mouse) I've noticed the video freezes for a sec from time to time.
I don't see that with plain MPlayer. Could it be SMPlayer/Dragon Player problem or KWin's ?

some video players put the video on pause when the window is minimized or looses focus. if that is the case, you can change that in the settings of the video player.

video freezes often mean that your system doesn't have enough resources to play the video flawlessly.

I'm talking about chops - milliseconds pauses while I'm moving around cursor or windows. It also "freezes" while scrolling lists in Dolphin or Konqueror.
Resources are well over the KDE4 requirements.

In theory I should watch the movie and don't work at the same time but I like to keep small window with a movie open while I do my work.

Have you checked this with VLC. You can switch to X11 video mode in VLC for best integration with KWin compositing (saying integration, I mean that openGL does not work properly for me with compositing on with my Intel onboard GPU)

To switch to X11 mode, open VLC. Goto Tools -> Preferences -> Video and change the "Output" to X11 video output. Save it and restart VLC.

Changing the driver to X11 in SMPlayer doesn't help at all (I noticed longer lock-ups).
I've nVidia QuadroFX card and checked also xv and gl2 driver.
No luck.
VLC on the other hand with default output set works without pauses. Time to switch

VLC is a great media player. I'd recommend it esp as it shifted to QT4.

VLC is fine but it doesn't open txt subtitles.

But the point is not to glorify VLC but to find what causes the other players breaks.

That probably is a GPU driver issue. I can't really help with that as I myself have Intel onboard not nVidia