Ramps 1.4 and extruder
Make sure that your heating device (resistor) is for 12v, not 24v (resistance is &lt; 10Ohm)
Check if the temperature readings that you get in your system are (at least roughly) the same as actual temperature (thermocouple with multimeter or any other device that can measure ~200C will help)
Check if connections are good enough. Low voltage at hotend is an indication of possible problem. ATX power wires are often too thin to transfer enough current. Consider replacing them with at least 2.5mm2 (AWG 14)