my citroen xsara picasso 2 ltr sx diesel keeps cutting out?

my citroen xsara picasso 2ltr sx diesel keeps cutting out. when i pull over and stop( hard to do without power steering ), and wait a few seconds i turn the key and it starts. the light on the dashboard stays on ,but turns off after a few days. some white smoke comes out the exhaust but not alot the aa man said it was a faulty fuel pump, but the garage said they could not find anything wrong with it. has anyone have any ideas. thanks