Heatbed overheat while printing

Hi. I got strange issue, i've build prusa i3 and get some successful prints, but today i started new print and on 4th layer get error "Error:Temperature heated bed switched off. MAXTEMP triggered !!", heat bed increase maxtemp (120C).
Restarted printer, in pronterface set bed to 100C and it heat and keep this temperature, but when when i started print it hit 105C (temp. fo 1st layer) and don't stop, increase to 115, i stoped the print, set temp to 0 in pronterface, but it don't turn off and continued increasing.
Thermistor is ok, i checked temp with ir thermometer.
RAMPS 1.4, marlin, mk2a heatbead.
In marlin BED_MAXTEMP is 120, in slic3r 105 for 1st and 100 for other.
p.s. Sorry for my english.

Sorry, change my question, just touched mosfet after 10 second of power on and it nicely roast my finger, heatsink (metal piece with hole) looks brown, i think it's burned.
What can be reason? I'll double check, but i see no short. And with what replace it?

An overheated MOSFET indicates the MOSFET is too small. Manufacturer error.

If it doesn't stop heating it might be toast already. Get an IRLB8743 and solder this one in.

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Generation 7 Electronics	Teacup Firmware	RepRap DIY

Quote
Traumflug
An overheated MOSFET indicates the MOSFET is too small. Manufacturer error.

If it doesn't stop heating it might be toast already. Get an IRLB8743 and solder this one in.

Thank You! Resolder to IRLB8743 , all OK, 2 hours in print and MOSFET has room temperature!