Arduino or Ramps problem? (no replies)

Printer running fine for almost 2 years. Smartrap, and very pleased with it.
Had to unclog the hotend nozzle, unfortunately, when I put it back together the heater element slid out of the block, the flex then broke off one of the wires.
No probs I thought, simply bought another heater element, 40w, and fitted it.
I replaced the thermistor at the same time. I have a bag of them, so it was the same spec as previously.

Problem:
The thermistor is reading an accurate room temp.
The hot end heats up (though not as quickly as it did with the old heater element).
The LCD is showing the rising temperature and the print temp.
When the print temp is reached, the printer starts the self levelling process
The hotend temp stays at print temp.
Then:
As soon as the printer starts the print process it appears that the power to the heater is cut,
The temp drops all the way back to around 140 degrees, and then seems to stay there.

I have reloaded the configuration software onto the 2650, but it is still the same.
All connections are secure.

Does anyone know what I should replace?
Would the new heater element cause this?
Has the Arduino blown a component?
Is the RAMPS board a likely cause?

If anyone can point me in the right direction I would be very grateful.

Many thanks

Rob