Hot end temperature varies during print
There could be a number of reasons for that behaviour:

Please check following items:


part blower/fan cools down the nozzle - stop the fan
material is extruded at high speed and takes the heat
loose thermistor (when the move occurs as it moves a bit internally) 
the power supply unit voltage varies - so temperature reading varies as well - measure voltage and see if there are significant drops
nozzle heater and thermistor cables can be loose as well (check screw terminals)
bed heater connection cables having not a good connection (check screw terminals)