I was having many issues with crackling on Ableton 9.7.1 running the Serum VST even though my CPU usage was only 20-30%. After extensive research I disabled the Intel SpeedStep and TurboMode technology which automatically increases the clock speed of the processor under heavy load. Ableton was not able to detect the clock speed change of the processor and as the processor clock speed changed based on load, it interfered with my audio playback.
To fix this you need to enter your computers BIOS outside of Windows.
Here is a snapshot of my workstation where I disabled SpeedStep and TurboMode Tech.