My KS 120 is losing power mid cut and only running 5-10% speed- what's happening?

The KS 120 REB has thermal protection built in, if the motor temp is too high it will go into a thermal protection mode. In this mode the tool can still be turned on but it will be running at a reduced RPM to cool the motor, the saw will reset to normal once the motor has cooled down.
Ensure the saw blade is the appropriate choice for the material you are cutting and the blade is in good condition.