2004 Vauxhall Zafira erratic idle, poor pull away and pinking

Most days when starting the revs shoot up to about 2000rpm before quickly dropping to about 500 (bordering on stalling) then back up to about 1800 before settling back to just over 1000 before settling to about 750 . Once driving, getting to about 60mph seems OK although slightly high on the rev side of things. But after that the engine seems to pink/slightly misfire. Being a mechanic, ive given it a full service and changed coil pack, plugs, all filters and even taken the throttle body apart and cleaned it. But to no avail. Randomly the EML will come on and give a code for 'mixture lean' could it be a faulty ICV (which is a sealed unit on the throttle body)???