Hi redtux and
A major two-fold problem with all notebooks is their compact size makes them very susceptible to heat issues and their nearly inaccessible interior makes them nearly impossible for normal users to clean and keep clean of heat trapping dust.
I suspect your CPU fan is doing exactly what it is supposed to do. When the CPU is taxed and gets warmer, the fan steps into high speed. This cools the CPU enough to allow the fan to slow down, which allows the CPU to heat up again.
I recommend you power off, open every cover and access panel, remove the battery and any add-in cards/devices, then use a can of compressed dusting gas to blast out as much heat trapping dust as possible from all the vents, cavities, under the keyboard, around the CPU fan, etc. Hopefully that will fix your problem. Then you might consider getting a Notebook Cooling Pad w/ext. power supply
. I prefer those with their own external power supplies so you don’t put more strain on the notebook, causing it to generate even more heat.