You need to PID autotune that heater. But first go over the heater and temp sensor connections and make sure everything is tight and heater and temp sensor are correctly attached to the hot end. Then connect via usb and enter M303 S235 C8 if you're printing with the PETG and it will return to you the correct kp, ki, and kd values for you to go enter into your firmware for the extruders heater. You can also do the same for the bed heater. For that you would enter M303 E-1 S60 C8 and wait for it to return your kp, ki, and kd values for you to go enter in marlin under the configuration.h tab. And then them heaters will work well. Also I like to apply thermal paste to the heater cartridge before i slide it into the heat brick. If i had one that was assembled dry I might be inclined to take it apart and apply some artic silver 5 or other cpu thermal paste. Because I use all E3D stuff all the temp sensors i use are also little cartridges and I also apply thermal paste to them before putting them in.