91 buick regal code 41 running rough

I have replaced the camshaft sensor...crankshaft sensor...water pump...some other sensor...have had these items checked and they are fine coil packs...emc module...timing chain...some wiring....fuel pressure good... a good 12 - 15 hours of work put into this car to find the problem. And all of this cost me $600 and the car still doesn't run right. At first it runs great then after 20 - 30 minutes it starts to spit and sputter and you dog it home. Dies sometimes but starts up again just no power. My mechanic gives up and now I have no idea what to do. Stuck with a car I have a lot of money tied up in and can't afford to get rid of it. Scared to take the car to someone I don't know for fear of a really big bill and still not have the car run. Anyone have any advice. Seriously ... other than taking the car out and shooting it.