Interesting overheating theory. Both the 3-way dimmers in question are each in their own individual box. They are both in a weird wall - it's a concrete wall that is fir'ed out with 2x2's. The box is a double-gang Carlon plastic shallow box (so it's only 1.5" deep) with a single-gang mud-ring on the front (i wanted enough room for the wires & dimmer). we used closed-cell spray foam insulation - in the walls - so it's pretty darn tight in there insulation-wise.