hi, my 54 reg 307 hdi xsi has developed a serious fault after being parked for 6 to 7 hours or more. When the car is started and I drive you can here the servo hissing the first two or three times I use the brakes, the pedal then goes hard and I have to press very hard to stop the car. At the same time the car seems to lose a lot of power as if the turbo is not working, but after maybe 400 or 500 yards the brakes return and so does the power. It also has an intermittent misfire some days and not others, there is no warning lights on the dash. I have had the car on a diagnostic machine and it shows no faults. Help, very annoying..