ESP8266 current spikes influence analog sensor
If you want to stabilize just the signal then you could add a shunt regulator just to the Photocell part of your circuit. Use a TL431 to regulate the voltage on the Photocell (the ref 2.5 V should be sufficient with a 390 Ohm resistor) from the 3.3 V regulator on the MCU. 



If the signal still varies too much, then you could try increasing the capacitance on the 3.3 V regulator output (I assume you added your extra capacitance to the 5 V supply).