Hot end temperature varies during print If I set temperature say 220 °C, printer heats up to it and it only varies +/- 0.5 °C under non operating condition. But if I start a print, there's a shift of +/- 15 °C.
I've already auto tuned PID parameters and when I run M503, the printer shows the updated PID values.

What could be the reason?
Electronics details:

Firmware used:  Marlin
Controller board: Printrboard rev D
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)