[Tannaton]

Check the anti-shudder valve which is the throttle vane type thing which bolts to the inlet manifold - has it jammed shut?

The valve is vacuum operated and closes when you turn the ignition off so the engine stops dead and does not peter out over a few seconds as the fuel system pressure falls.

I've cut mine out and sealed the resulting hole with a nut & bolt & PTFE tape (also blanked the vacuum feed). The shudder on switch off is barely noticeable.