i have a 2001 pontiac boneville 3.8 . Never had a problem until recently it started have a quick hard miss only once and then would run fine, but i noticed that the rpm gauge fell to zero. I could cut off the engine and restart with it working again. It has been a couple of months now and it has started cutting off going down the road without notice ( completely dies). It cuts off just like a light switch. After a few minutes it will crank and run again for days without a problem. Just when you start to forget about it , the same thing happens again. I have replaced the oil pressure switch and the fuel pump & filter. What else should i check .