Prusa I3 Temperature Falling

[printbus]

...hot end immediately starts cooling, like it has been shut off...

Using either the LCD on the printer (if it has one) or the host software you are using, can you confirm whether the "set temperature" for the hot end stays at 210 when the hot end seems to be cooling off? If the set temp drops off, the printer is doing what the slicer is telling it to do in the gcode. If the set temp stays at 210, it could be something like intermittent connections at the heater or in the wiring, or printing way too fast for the hot end to keep up with.

EDIT: If the set temp drops off, another possibility is that the connection to the thermistor in the hot end either shorts out or opens up. Most if not all firmware will detect this as an overtemp or undertemp error condition and shut off the heater circuit.