Violent jerking and cutting out in Peugeot 106 v- reg?

Hello, my car (Peugeot 106 Zest 3, V-reg) has a problem my mechanic can't solve. When I have started it up recently (though not every time), the engine whilst idle makes a loud irregular noise, and vibrates irregularly. Then when I drive it, luckily at speeds under 40 mph, for the first 5-10 minutes it goes through phases of jerking violently, as if I was stamping up and down on the accelerator, then invariably cuts out. I don't feel safe driving it, but my mechanic, having replaced my head gasket and water pump at great expense last week, doesn't have any idea. Can you help?