i've experienced this very problem on several laptops and finally found out that even a thin layer of dust on the small cooling ribs of the laptop cpu heat sink will cause a decrease of cpu speed due to cooling efforts of the automatic cpu speed control in modern laptops.
it usually occurs some time after booting and using your laptop and of course is os-independent. it may also occur without system alert or anything.
in order to avoid waiting terms of consulting the manufacturer's repair-service i routinely open my laptop and deliver the cooler from dust.
if you choose to do the same, be aware that you may have to completely remove the heat sink from the board so you'll need heat sink paste and a certain amout of sensitiveness.
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i've experienced this very problem on several laptops and finally found out that even a thin layer of dust on the small cooling ribs of the laptop cpu heat sink will cause a decrease of cpu speed due to cooling efforts of the automatic cpu speed control in modern laptops.
it usually occurs some time after booting and using your laptop and of course is os-independent. it may also occur without system alert or anything.
in order to avoid waiting terms of consulting the manufacturer's repair-service i routinely open my laptop and deliver the cooler from dust.
if you choose to do the same, be aware that you may have to completely remove the heat sink from the board so you'll need heat sink paste and a certain amout of sensitiveness.