It sounds like the ceramic(plastic?) socket in the dome melts or something, causing a short? Are the ends of the cords baked and blackened? It makes an actual POP-ing sound, and is there soot on parts of the socket or fixture after it pops? The soot comes from burned insulation.
I can only assume one of two things: 1)that the fixtures you are buying are substandard with plastic sockets and/or no vent holes in the hoods, or 2) the cord is wired directly to the ceramic socket. There are usually 3 or 4 vent holes clustered around the top of the hood so the hottest air can escape. That causes a flow of cool air going up inside the dome, around the CHE, and out the "chimneys" which are the holes in the dome. The cord needs to have minimum #16 wires.
If your domes don't have the vent holes, you can make them by drilling four or five 5/8 inch holes about an inch below the top of the dome. If the sockets have been plastic, hopefully you can retrofit with ceramic sockets. They are common in hardware stores. Hopefully the ceramic sockets have six inch heat resistant wires (pigtails) already in place.
Ceramic sockets shouldn't be wired directly to normal electric cords: The heat can melt the insulation allowing the wires to touch each other causing a short. There should be 2 pieces of about 6 inches of heat-resistant insulated wire (they have fiberglass-like woven insulation and stiffer wire) attached directly to the socket, with the cord attached to the ends of the heat-resistant wires. Don't solder this splice because the heat could melt the solder. Heat-resi9stant (ceramic) wire nuts are commonly available if you call around.
250 watt loads should be wired(at the ends of the pigtails) to #16 or thicker cords. #14 cord is safer. Lamp cord is good for maximum 100-150 watts.
Does this help? (-: