What is this fluctuation I get when I use the DC electronic load?
It could be common-mode noise.
You stipulate you have a buck converter, guessing it is a DC/DC converter which implies you have a power supply powering your buck.
120khz is typical from a switched power supply and they generate usually quite a bit of noise and ripple.
Try to add capacitors and also common mode choke at the output of your supply, eventually a capacitor multiplier, and see if that reduces the noise.