Renault scenic flat spot?

Having recently bought a scenic 1.9dci 02 reg, I find that when moving off from standstill the car seems to be flat, then after say 10 yards seems to pick up, there was a K&N filter fitted but i have reverted back to the original,the symptoms were the same even with the K&N, the engine sounds really sweet, has anyone got any ideas,Thanks in advance if you do,