Right, had this issue with the Omega for a few days since I had it back, basically a missfire, no power, sounds like a scooby and is generally rough as toast.

So thanks to Stevemoss for providing a coilpack and leads, which after some scuffed knuckles, lost loose change and some blood I can now say the new coilpack is in, and it's made little difference, car does seem a little better, but given the condition of the old coilpack and leads I'm not surprised, the right bank's rocker gasket is leaking too as I took the plugs out to discover them oil soaked.

So now I'm a bit lost, do I continue to try and see if it's the cat's? Or is something else at play here.

It's hard to explain how it's running, but 'rough' would be an adequate term...

Did you change the spark plugs?