Just a suggestion (that is, if you don’t buy a proper dust collector), get a dust deputy or some other pre-filter and also use a material “sock” over the internal paper filter. Then, when doing long jobs, turn the vac off for a few seconds and give it a few knocks.
My entirely unsupported hypothesis is that the really fine dust clogs up the filter significantly reducing the cooling airflow through the motor. Turning it off occasionally and giving it a few thumps, allows this dust to fall away from the filter and allow a little more airflow. Because the dust I’m talking about is so fine, it’s often not really noticed when you open the can, and by the time you’ve opened it, much of the dust has already fallen away anyway.
The solution as mentioned above, is to get something designed for doing this task, however if that’s not an option, my suggestion may mean you burn out the bearings every 18 months instead of every 12 months.
Just a suggestions…