The phenomenon you describe reminds me of a strange bug I once experienced when running Quake3: The game ran smooth, but every few seconds, it just "froze" for a short period of time.
The reason for this was that I had unplugged my GamePad, but Windows thought for some reason that it was still there. So I guess Windows tried to "poll" the GamePad in regular time intervals, which caused the stuttering. After I removed the GamePad in the Control Panel, the problem was gone.
However, as you wrote in the German forum that you also tried examples that do not use any SFML-input stuff, this explanation might be unlikely. But as you don't have problems with the NeHe-examples (which are graphics only), I think this problem might be caused by some SFML-code that is not graphic-related.
Have you tried to strip down an example where the problem occurs? If you have some sample code, try to remove all Event-Handling and Input-Related code and try again.
You could, for example, remove the event-processing in Wolfram's sample code (you won't be able to close the application if you do that, but as it's only thought for testing, that might be ok
).
