2003, 520iSE petrol with 50k on the clock. The I-drive reported an engine fault – drive at reduced speed to avoid damage to the catalytic converter. BMW could not look at it straight away so I removed each spark plug to check. One of the six seemed wet. After a clean it was still the same. Managed to do a OBD diagnostic on a laptop and it said cylinder 5 misfiring. I replaced the spark plugs (under £15, I can’t image what BMW would have charged!) with no difference. Next step was to change the coil pencil since 3 of the six had been replaced with Bosch equivalents at some point. Misfiring problem cured. Now looking at the rev needle it seems to sometimes drop approx 50 revs every second or so even when the engine is warm. Should I also change the remaining 2 factory fitted coil packs or is something else probably wrong? Also starting the car anytime warm or cold with the head lights on, the light dims to almost off but the battery has never lost power. Could this be an issue or is this normal? Cheers.