ubuntu 14.04 periodically becomes VERY slow

[robert]

Hi there,
one of my boxes with ubuntu 14.04 periodically becomes extreamly slow.
the desktop  halts for periods of maybe 10 to 30 seconds. the active 
windows just grays out.

If I have to running in a window during such periods, I can see nothing 
special. No process runs to more than a few % cpu.

any idea, what could be the reason?

thanks
robert
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