Cadillac Repair: 4.6L northstar problems, fuel pressure gage, mystery car

Question
yes i have had this car to a caddy specialist and four other shops and spent over twelve hundred dollars no one can seem to figure it out.it stalls out of no where or going down the road it will skip a beat or just plain shut off.sometimes it will start back up and sometimes not.then you wait and it will start and imediately shut off repeatedly.sometimes it will start and you can hold the throttle at a slow steady speed in park and the engine will just automatically set there and rev up and slow again over and over it will do that and when you let off the throtle it will shut off.it has had a new gm map sensor a rebuilt one,it has throttle position motor replaced three ecms under the dash.everybody says maybe cam or crank sensors.i'm basically tired of replaceing parts at three hundred a pop at this shop and that shop and it not be fixed.the cadillac specialist says he could get it do those thins but everyhting checked out.he said it was a mystery car.i love the car please help thanks larry

Answer
IS or HAS the CHECK engine light ever illuminated during this problem ?.

The first thing you are going to have to find out is if your problem is electrical or fuel related.

I would install an external fuel pressure gage on a long hose and attach it to the outside of the windshield and watch the fuel pressure when this problem starts to happen.