engine shut down by itself after hearing " clank" sound

motor shut down for no reason
new radiator, water pump, all belts and hoses, thermostat, plugs & wires, new head gasket

no water or coolant on oil dipstick - oil level correct but burnt smell
trans fluid ok
no leaks found
water pump spins freely
thermostat in-tact and manually functional
plugs clean w/o oil or moisture
coolant level down 1/4 gallon but no signs of any leakes
shop says the computer shut down @ 57mph. and a temp. reading of 275 --- had been driven only 5 miles
plug wires were melted to valve cover