Dave,

Is it not possible to scope the coil packs when the misfire is present. Assuming the plugs are OK you can then eliminate (or not) an ignition mis-fire. You are then down to a fuelling or mechanical problem. By under-taking a 4 gas emission check you can then determine whether you are looking at a fuelling or mechanical problem. If this beast has hydraulic valves, you might have a valve stick problem which can be eliminated by using FORTE treatment.

Regards

Pete M