HOT END AUTOMATICALLY SWITCHES OFF DURING PRINTING

Prusa i3 Sunhokey
Ramps 1.4 + MEGA / MK4 Marlin v4.3.6
Repetier Host - Cura Engine / Slic3r

Heat bed temp 55°C
Hot End temp 190°C

I printed a 20x20x20 calibration cube with no problems.
The printer reach the temp perfectly, infact I have tuned the PID for both components.

But when I go to print any other object I encountered a problem.
The print starts and deposit on the plate the first layer normally, also the second and so on.
After random (30-45 min) time the hot end automatically switches it off during the printing.
Obviously lowering the temperature, the extruder stops because the minimum operating limit is 170 ° C

BUT continuous printing and the z axis rises layer by layer, so the hot end moves empty

Could someone help me?
I can not tell if it's a hardware or software problem

It sounds like a hardware problem. Check that the heater cartridges connection to the control board is good. if you have a multimeter you can check the resistance over the cartridge while flexing the wires and see if it changers significantly. I would also recommend you replace the heater cartridge if the problem doesn't go away.

I have had this problem too and solved it by soldering the crimp connections where the wires meet the heater.

The absurd thing is during printing, the hot end and the heated plate switch off.
During printing, in the manual control panel of the repetier host I press the buttons for the hot end and the plate and they turn on and reach the set temperature.
This means that I have to be perpetually in front of the PC to monitor that they do not go out.
Punctually after a certain period of time they turn off and to solve it as long as I manually turn them on from the repetier host control panel