I would echo Hannes' thoughts here - are you sure you're using enough revs? These are modern engines and don't appreciate being lugged around at low revs, remember they are designed to spin up to 7k revs or more, and the sweet spot for power is in the 3-4k region.
Also as Hannes says, put decent petrol in it, none of this supermarket rubbish, Shell V-Power or BP Ultima is what your P&J deserves, nothing less.
But if you're revving it and putting good petrol in, then maybe a RR session would be in order as Eddie says.
