GeForce FX Go5200 performance up and down

Hi there!
I have a little problem with my dell notebook and it's graphics card.
It's not that everything is going too slow, no, the problem is that the
performance is sometimes going so high up
that I can't play properly any more and then again it slows down until
everything is unplayable.
I've the impression that this problem started only with the last update of
the drivers (their date is 8th of January 2004, their version number is 6.
something although thanks to DELL's impressive expertise in supporting users
the version info still says : 4.5.6.8).
Has anyone experienced similar prob's? Or any idea for Solution?
Thanks,
Manuel