How can I fix a flat spot on my renault megane 53 plate?

Hi, I have a flat spot on renault megane 1.6 53 plate. Its worse in first gear, and can even feel it when ticking over, or changing gear. Quite dangerous when pulling out of a junction as you lose power. Been in renault garage, and shows no faults. Already had a defazer pulley repaced but still no help! Anyone any ideas? Many thanks......