Are you powering it through the RAMPS?

I found that the polyswitch hasn't really got a high enough spec. to cope with both the hotend and the heatbed. It is very borderline. The polyswitch was getting super hot and the wires from the power supply warm to the touch. All I did was desolder the polyswitch and replace it with a 16V 15A spec. one like RHEF1500-2. Some people have found that they have fried the MOSFET and have replaced it with an IRLB8743 MOSFET as well on their RAMPS.