my sister had the same problem, loss of power and the engine cutting out whilst driving, and in the end got rid of the car, she bought it from a car warehouse type of place, and they could never fix it, my personal opinion is that they wouldnt pay for vauxhall to fix it, because it was an expensive job. she had trading standards involved too. but lost all faith and traded it in for something more reliable. the car even cut out as she drove into the garage when she was picking up the new car, Vauxhall dealer should be familiar with this problem, as i beleive its a common one, how often does it happen, if its regular, get the dealer to do a "LIVE" diagnostics test. ie drive the car whilst someone is looking at the live readout on a laptop.
Hope you get it sorted,