You figured it out before I had a chance to chime in The mosfet heatsinks is an old and infamous issue for the RAMPS 1.4. Even worse (and you will love this) is that the mosfet channel for the heated bed is one of the common failure points for that board. So eventually it will simply stop controlling the heated bed and you will be replacing the RAMPS. It should not happen soon or quick, but it will eventually Almost a wear item. My original machines that I have had for 5-6 years have each had the electronics swapped out once. The new ones have not yet had that problem.