Two things I would look for... Corrosion in the fuse box in the engine bay, and check the Coil packs to ensure they are not breaking down... you will normally visibly see swelling or cracking on the coil packs.
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When working with GEMS vehicles. Never try to locate a misfire by pulling plug leads off - pull the injector connector instead. Otherwise you can blow up the ECU.
Secondly never swap ECUs between vehicles to test them, because both vehicles and ECUs may stop working, and you'll have to have them all reset by a franchised LR dealer (ouch!).