I had a diagnostic test on my vauxhall zafira as had the same problem as you just described, very dangerous. the test proved inconclusive i have since read articles on the internet from watchdog saying it is a common fault in some models of vauxhall, the fault being in the fuel pump and/or fuel regulator. I am personally seeking vauxhall to pay out as it appears to be their problem.
Gemma
April 2010