Hi guys I'm new to this and new to owning a ford (usually Honda) I recently bought a '03 plate st170, 100k miles, completely standard. Engine management light was on and I was told it needed spark plugs... The car misfires sometimes up to 3k and sometimes through the entire rev range, there is a knocking noise coming from the passenger side of the engine bay when the car is idling, also misfires at idle, the car had no imrc box so I bought one, it made no difference, the cable pulls for a few seconds then releases, I've been told that is a faulty box, but the strange thing is when I reset the ecu (disconnect battery) the engine management light goes off and the car runs smooth and responsive (still has random misfire and juddering) but then when i turn the car off and back on again, the engine light is back on and the problems start again, surely if the car needed new coil pack/plugs then the problem would be a consistent thing?? I have considered it could be a problem with the fuel pump cause my tank some how fills up randomly while I'm driving, but I really have no idea what to do and I don't want to throw parts at it, so I thought I would ask the people that know, any help would be greatly appreciated, many thanks.

Joel.