Prusa i3 heatbed MK2a can't reach 110ºC and print issue

Hello, I bought the a Prusa Mendel i3 with the mk2a heatbed. I program the bed to reach 110ºC to print with ABS but it can not get past the 103 -104 °C. I have checked several times the connections and the welds. Even I solder better one wire that seems wrong weld. But no change.
I have also noticed that the mosfet that controls the hot bed is so hot and when the bed reaches 100 degrees it goes down to 98ºC, and after a moment it goes up again until it is able to pass the 103-104 ° C. The mosfet is a p55nf06. The power supply provides 12V and 30 amps.

I'm using an arduino + ramps 1.4 with marlin 1.0.0 and repetier-host.

In order to print I had to lower the temperature of the hot bed to 103ºC so that it starts printing. The figure is a small empty cube and I attach it.
And the result is pretty good but the 4 outer corners curve inward in the middle and the walls also curve.

I would definitely use a heatsink and fan to cool the power mosfet for a heated bed!
I also use a bit of corrugated cardboard as a sort of 'insulation' underneath my heated bed, and that seems to help out a little, too.
The problem with your cube is probably heat/cooling related, have a look at this: [Reprap wiki page]

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Thingiverse Profile

I use a heatsink to cool the mosfet and I used a bit of insulation under the heatbed. Now it's work perfect and reach 110ºC perfectly. Thank you.
Now I will solve the printing problems .

I'm glad it's working! Best of luck to you on the rest of your calibration!

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Thingiverse Profile

To speed up the "warm up" I place a piece of cardboard on top of my glass as well. Cuts down the time to heat up significatly.