Originally Posted by Throttle Jockey

i've heard of this before and it seams to fit your symptoms. maybe check the egr valve. if it is sticking in the open position (like it would be when you're going highway speed) when you let up off the gas pedal, you basically have a huge vac leak and you are pulling exhaust gas back into the engine at a reduced rpm. air fuel ratio is off and engine dies. egr valve slowly closes when the engine is not running and threrefore the vac leak goes away, starts back up like nothings wrong until the next time it happens.