Voltage controller is on Mega board. RAMPS itself do not have any voltage regulators it only feeds 12V through D1 to Vin pin on Mega. Mega can be powered form USB or external power supply, but if you have connected both it takes power from external power supply and uses on board 5V regulator to step down 12V to 5V.
[attachment 44176 ArduinoMega2560_R3_Fronte.jpg]
One other way to test if voltage regulator is working alright is to disconnect Mega from RAMPS then feed 12V to Vin or DC input jack, then measure its 5V rail. If it can not produce 5V then voltage regulator is toast. If it reads 5V try loading 5V rail with simple resistor, I would not try to pull more than 500mA (V=I*R, so for 500mA 5V/0.5A=10Omh;2.5W resitor.) If it can handle the load then RAMPS board is causing problems not Meaga.
As for mosfet overheating, which one and what have you connected to RAMPS board?
[attachment 44176 ArduinoMega2560_R3_Fronte.jpg]
One other way to test if voltage regulator is working alright is to disconnect Mega from RAMPS then feed 12V to Vin or DC input jack, then measure its 5V rail. If it can not produce 5V then voltage regulator is toast. If it reads 5V try loading 5V rail with simple resistor, I would not try to pull more than 500mA (V=I*R, so for 500mA 5V/0.5A=10Omh;2.5W resitor.) If it can handle the load then RAMPS board is causing problems not Meaga.
As for mosfet overheating, which one and what have you connected to RAMPS board?