FESTOOL The armature should in most cases be bad replace it with new carbon brushes together.

BUT
If its a battery tool it can have the same kind of problem but little other diagnosis if the armature looked colored then thereisanother aspect when the sound also is hearing sharp or shrill then only replace because the armature collectors are eaten the brushes by thermic shorting , much people think they have bad capacitance problems with the expensive batteries but much are uying new batteries but the tool should have not the power like before most of the machines have bad armatures in a little percent of them the powerswitch can have the problem, IF li ion batteries are used beware on the sounds and rotation of your machine if it sounds sharp ( in no way smooth) and the tool has no power replace allways the armature but if sounds good and smooth borrow temporarily a new battery to see if also the work force is still present, then you can possibly go for a new battery, by problems with brushless tools the electrical pcb assemblys of the switch is having the highest priority not the battery.

That is common with power tools when being used. If it is a cheaper brand, it may have a weaker motor in in.