Cheng, I had a similar problem a few weeks back and it got worse for a couple of weeks, until I was unable to satart the car. I checked for pressure at the fuel rail and there was none. So I checked the fuel pump relay and it was Ok. The fuel pump was bad. It is my second fuel pump failure. First was at 87k miles, second now at 182k miles. I figured that the pump pressure and flow will fluctuate as it approaches the death, which explains the sudden and occasional power losses. And yes, my check engine also came on, but stayed on. The code was 231, perhaps because of the fluctuations on pressure and flow. Everything back to normal after I replaced the pump (OEM). By the way, mileage had been deteriorating. Back to normal now.