Heating problem during printing (1 reply)

Hi,
I am using Marlin 1.1.8
Heating is just fine when the printer is idle and I tuned the PID parameters. I also tried extruding a quite amount of plastic and temperature dropped just 4 C before increasing back to the target. However, as soon as I start printing, nozzle is cooling and it cannot restore the temperature back. It goes back to normal when I paused.

Can you guess what is the problem?