still struggling with the trusty c5.It doesnt appear to have a fuel pressure problem.Took it for a long road test with scanner plugged in (sykes acr 4} with fault appsrent- shudders under load up hills mainly ,fuel rail pressure and lamda readings stay constant.I have changed the high pressure pump,coil pack and plugs previosly for other faults.Did replace VVT hub,engine sounds perfect now.Running out of ideas now .No one else can/wants to fix it !