I have a 2009 C5 2.0 HDI (X7), approx 82k miles This can be erratic but originally on mid-heavy load I would get an engine vibration as if a misfire. This has slowly got worse and yesterday the vibration was slight on just touching the accerator to keep speed and severe engine shuddering on mid-heavy loads. If I brought it to over 3,000 rpm it was drivable. This morning when I was driving to the garage for investigation, the engine vibration was just detectable and no more. The garage read no errors and all appears to be Ok, they also check the software was the latest for this C5. As part of my own Lexia investigation previously I could not find any errors (as the error log was full because I installed LED number plate lights! reporting short crcuit). I installed a EGR bypass, no change, still shudder. Checking the Lexia log now reported "Excess air"? and "EGR failure/blockage", so the EGR blocker was removed. The C5 was full serviced with new fuel filter 2months wth no reports of low fuel pressures. Also treated with Forte Diesel and DPF filter cleaners. The only other interesting point is that fuel usage is less. This has moved from approx 50MPG to 60MPG! Does anyone have any ideas what else to check (that isn't too expensive)? I am lost on