Clio 1.5 dci Expression

I have a new clio which I bought 10/2007. It has been 'lumpy' since purchase and the dealer says there is nothing wrong with it ... but it 'misses' occasionally and is very lumpy for a diesel car. Does anyone have experience of this problem and any suggestions? It has also let us down a couple of times by not starting ... the AA have been called each time and cannot find a fault! Maybe there is something more complicated causing the 'lumpiness' and occasional failure to start > Any suggestions??