Heated bed out of control!

Hello. I'm using a RAMPS 1.4, the Marlin firmware and Repetier-Host for printing.

I set up the heated bed max temperature for force stop in Marlin to 150 ºC and the working temperature in Repetier-Host between 105-100 ºC (for ABS).

When I start printing everything works well, when the heated bed reaches 105ºC the printing starts and the temperature is stable at 105, but after few minutes of printing the heated bed starts to heat again without control, reaching 150 ºC. Obviously the firmware stops the printer at 150.

I don't know where is the problem:

[i.imgur.com]

And I can't stop the heated bed until the printer finishes.

Thanks and sorry for my bad english.

I hope you dont use pid control on heated bed.
In the picture i see the fan is set to on. What pin is allocated to fan in config.h or pins.h for fan, could it be same pin as the heated bed? Or you do have a fan and it actually works properly?