Engine cuts out

I am having a problem with my 1998 venture. After driving it for a while (at least a half hour) it will start to cut out. It starts at first when going around 55 mph. and it is very suttle at first and sometimes hardly noticeable. It will though ventually get so bad that the car is almost undriveable and will eventually just die off. If I park it for a few minutes, I can start it and drive it but not very long before it starts running real bad again. If it sits overnight, it will start and run good for at least a half hour before it strts to run bad all over again. The mechanic I took it to changed the cam shaft sensor and the crankshaft sensor because of error codes he picked up from my cars computer chip. Neither solved the problem in the least. Please help!! My mechanic (the local chevy dealership) hasn't a clue to what is causing this

Tape a fuel pressure guage to the windshield. When the engine dies do you have fuel pressure ? Test for spark with an extra spark plug. Do you have spark ? Then you can go from there.