Hi, try using Shell V power as I had the same problem. I had changed the plugs, leads and coil pack, it ran better but still occasionally had a slight missfire.Today I needed petrol and only use Shell regular as I don't trust supermarket fuels. I gave Shell V power a go and believe it or not the power is back and no missfire.! (I WAS ACTUALLY GOING TO POST A QUESTION ON THIS FORUM TO SEE IF ANY ONE ELSE USES IT OR HAD THE SAME PROBLEM AS ME),when I saw your plea for help. So I hope this help's.
Cheers.