Hi, my Td5 1999 starts and runs fine but the last few days ive noticed it seems to be misfiring (feels like a murmur if that makes sense), this happened when driving around 50-60mph, then when I press accelerator I notice it puts out blue smoke then runs fine again. I intend to change fuel filter this weekend to see if that sorts but im wondering does it sound like anything else to anyone. engine has 220K miles.