Problem with the themperature

Hi, I have a problem with my Prusa Iteration 2... This problem is that the temperature up when the hot-end stop and when hot-end start the temperature decline...

For example... the temperature arrive at 240ºC and the firmware stop to keep... then, the temperature increases. When the temperature up the temperature decreases 30ºC...

The difference is always the same...

I saw that 12V expansion pins start with 5v of usb? O_O

Yeah Finally I resolved the problem... if the cable of ground is long the valors change some

And 12V in the wiki say that is a bridge that I must cut.