2001 Hyundi Elantra w/~ 128K miles. Problem started 1.5 weeks ago. While cominmg home from work, still cold engine, the cars engine. Able to coast to side of the road, tried to restart car. Did not start. Sat on roadside for approximately 5 minutes, car started. One week ago, drove to store. Stopped to shop, got back in car, almost same symptom. Car would not start. Tried several times to start engine, no dice, engine just cranks. Wait 5 minutes, car starts. 2 days ago, stopped top purchase gas, got back in car, would not start, wait 5 minutes and the car then starts. Today, while drive to pick up some dinner (cold engine) the engine started to buck, but did not just shut off, as in the prior 2 cases. Car kept going. It also seems like the car is getting poor gas milage. All dealer prescribed maintence has been done on the car up to and including the 100K service. Any assistance would be appreciated. Thanks in advance.

This is only a guess. I had a similar problem (intermittent) and it turned out to be the engine coolant temperature sensor was going bad. The EMS (Engine Management System) uses all kinds of sensors to regulate fuel-to-air mixtures through a range of temperatures, engine & vehicle speed, and transmission shift points. When the engine is cold, the fuel-to-air mixture is greater, and when the engine is warmed up/hot, this mixture is reduced. What was happening in my case, the ECT (Engine Coolant Temperature) sensor was telling the ECM (Engine Control Module), that the engine temperature was at -40 degrees (F.), so the ECM richened the mixture & was actually flooding it with too much fuel. This sensor might also be telling the computer that the temperature is higher than it actually is, then the computer would be adjusting the fuel (LESS) causing the engine to NOT recieve enough fuel. You'll want to puts your hands on an OBDII scanner which not only reads codes, but also displays engine current/live data. You'll want to monitor the ECT sensor to see if the temperature fluctuates wildly (i.e., from -40 deg to like +260 deg., etc.). The best time to check this is when the vehicle starts misbehaving as you described.