[AndysR]

If your 8 psi down at idle just think how much it could be down by when the fuel is in demand at higher engine speeds and the fuel pump can't provide a reserve of fuel. I have known of vehicles which appear to run fine at idle but when the engine speed increases and fuel is in more of a demand a poor fuel pump can't cope and misfiring and lack of power occurs. Bit of a silly question, because I would put money on it that you have anyway, but have you changed the fuel filter? Also did you fit it the right way round? Not that I am sure you can physically fit it the wrong way round on the 850 as I haven't actually looked at mine but a partically blocked or incorrectly fitted fuel filter could also give the problems you are experiencing.