The voltage level fluctuates whenever ESP8266 receives signal
The ESP8266 is notoriously noisy, chiefly because the cheap boards the ESP8266 is commonly sold on don't implement any form of power filtering.

So you really have to add your own.  As I had to recently when adding an ESP8266 board to an audio system.

For that I came up with this schematic:



VCCA is your main incoming power.  VCCD feeds the power into the ESP8266. The motors are connected to the same point as VCCA.