2001 Focus ZTS-Engine suddenly shuts off

I have a 2001 Focus ZTS with 170,000 mi. At low speeds the engine suddenly shuts off. There is no hesitation or sputtering, it simply turns off. It will start right back up however. Last tune up was at 100,000 mi. The Ford dealer said it is impossible to know what to fix without the check engine light on. The engine light never comes on when this happens. I'm at a loss of what to fix without replacing part after part, which could become very costly.