Diesel engine sometimes quits running during long drive, starts up again in 5 - 10 minutes.

Have had vehicle only 8 months. Runs great otherwise. Long drives of more than 3 - 4 hours continuous is when engine suddenly quits running. But starts up after 5 - 10 minutes of waiting. Previous owner mentioned having to install an extension device for some kind of electrical control to place it off the top of the engine and away from it to keep the "control unit" cool. Not sure what this "control unit" is called. It is approximately 3 x 3 inch square.