Hi Scatz - well here goes my comments
1. Erratic idle would make me think the engine wants to cut out but the
compensates by either adjusting the fuel amount or when the fuel is injected. It also can adjust the ignition timing - it the only things the
can vary That would make me think it was a fuel control problem otherwise the revs wouldn't rise or fall. In this situation I would suspect weak mixture - either lack of fuel or too much air thats why I suggested a leaky inlet manifold. Lack of fuel could be a number of things - dirty filter, poor fuel pump, duff fuel presure regulator or blocked injectors. Have you tried injector cleaner yet.
2. If a
were failing they would tend to be worse when hot and the engine would either miss a beat or judder slightly when idling.
3. As the engine appears to miss when driving at a constant speed it would lead me towards are duff or faulty Lambda sensor - it may be worth getting the emissions checked by a decent garage who may be able to give you Lambda sensor readings. If the Lamba sensor is over compensating it could lead to under or over fuelling.
4. Whilst the plugs are all new have you checked them for colour after a run - they should all be the same.
The slight back fire is the result of unburnt fuel entering theexhaust - either because the engine is over or under fuelling.
5 What I mean by hunting is that during idle the engine sound pulses - tis would be the symptom of too much fuel.
Although I have gave no definitive answers - if it were mine I would get an emmissiions check done - good luck