plugin-container.exe disabling Windows cache?

Seamonkey up to date (2.5). WinXP 2GB

Sometimes the system becomes very very slow and the harddisk 
fiddling. Repeated loading of the same (small) program lets the 
disk get busy, as if there would be no caching anymore in Windows.

I found that when I kill just plugin-container.exe  , then this 
spooky OS state ceases completely. Yet plugin-container.exe does 
not consume significant amount of the Memory or CPU in the task 
manager, just some 50 MB / 0..1% CPU

(probably just Flash and pdfs have been used during such sessions)

Is this a known problem?