Vauxhall Corsa car cuts out randomly - driving or starting up.

Basically this is a really annoying problem, my car cuts out instantly at random times, either when I start it, or when Im driving, but always at random times, I could drive 60 miles without any problems but later in the day without fail ramdonly it will just cut out, and when I try to re-start it it won't turn over, but by the time the aa get to me it works again. The aa said it was the crank shaft sensor the first time, so got that replaced, 2 days later cut out again, it and wouldnt start this time, aa man got it working again by cable tying the coil pack connector plug as it was broke said there was a bad connection, I had this repaired drove for miles after it was repaired and it randomly cut out again, it's been plugged into a machine at the garage and no faults logged. No one knows whats wrong with it, the aa said it's not a fuel fault as they chacked the pressure, no one can fix it as it always works fine when they look at it. the aa man who witnessed it cutting out said it cuts out so quickly, as if someone turning the key to switch it off. any ideas?