I had the same problem for a long time and replaced the fan clutch, thermostat and 4 trips to the dealer. It was low in coolant and just to be safe we did a sys flush and problem solved. It would do it in the city from stop light to stop light or while turning. It turned out that every time the car slowed down or I took off the level of coolant would shift and hit the level sensor and trigger a low fluid/high temp and car went to reduced engine power and misfire heaven. Never did it on the highway because I was traveling at a steady level and there was no pitch and dive motion to move the fluids and trigger the sensors. Same thing happened with the "mystery beep" that went off but never registered anything on the info center. It was always low washer fluid, but it was so brief that the info center never displayed it.