Strange problem with heated bed not reaching temperature

[Glentved]

I have a RAMPS 1,4 with repetier 0,91 firmware
My heated bed has been working sorta.
The problem is that I am using a 3mm stainless steel plate for heated bed.

If i put a box or something onto the bed, the printer could heat to 100° C in ~30min.
But when i removed the box, the powersupply could not deliver enough juice to keep it hot, the temperature was falling slowly.

So I have increased the vorking voltage from 12V to 17V.
It works and the bed heats fast, but not past 60 degrees, then the output shuts off and never on again.
I dont get any error reports. All seems to work fine. The same error occurs when I control from repetier host or manually preheat. So the problem must be in firmware.

I dont know where I should look for a problem.

Before the voltage change the printer worked and I could print ABS.
I use 17V for the heated bed only, the rest is 12V (steppers and hotend)

I hope someone have a suggestion.

[Glentved]

Sorry I can see that I have posted the wrong place.
I have solved the problem.

The mosfet switch got too hot and desoldered itself from the ramps.
I will replace it by a better one or attach a heatsink + fan to cool the entire ramps board