How can i fix my Renault megane RT R reg 1.4?

As i am driving along, when i come to a stop at traffic lights or a junction and keep my foot on brake or something, my car starts to shudder, and its not the idle thing. and 2 times this has happened is when i just start to drive it down the road a bit the engine will just cut out completly and i have to restart it again but thats only happened twice in space of month.and another problem is that my cat on the car sounds like theres loads of stones going round in it. could someone help me solve these problems im not very good with cars.