It could be several things, from firmware to hardware.
Check the Mosfet on the board and see if it's getting too hot to the touch (it might shut down due to temperature)
Power supply: not enough power?
Wrong thermistor setting giving wrong readings?
Bad heater cartridge?
Loose cables?
Power settings in firmware limiting power to hot end?
Start with the simplest possibility (loose wire) and work your way up untill you find the problem.
Regarding the capacitor, the only thing that comes to my mind is noise cancellation and voltage stabilization? Put between +and- coming out of the power supply.
Edited 1 time(s). Last edit at 12/01/2014 07:47PM by ggherbaz.