Holden Commodore 2003: Engine troubles

My 2003 VY Commodore V6 lurches almost as if the engine is being starved of fuel or air. When driving the speed will drop off all of a sudden and so will the revs, it has also stalled on me before as the speed and revs gradually got lower. I took it to a Holden dealer and they found a blocked fuel filter and replaced it; they also did a fuel pressure check and found that was ok, and checked for error codes but found nothing. But now I am having the same problem. What is wrong?