Why does my 1996 Holden Vectra Opel Vauxhall keeps stalling when decelerating it will restart when key is turned but cuts out whenever slowing down?

Start the answer off. It's probably the idle control valve that needs replacing. It's a silver cylindrical device near that's easily replaced yourself. You can get a new one for about £120 from Vauxhall or a second hand one for about £20-30 from a scrappy.