Stilo tickover

Evening all!
I have a Stilo 1.6 5 door Active. It has been reasonably well behaved for the last four years but two days ago started running badly with
erratic tickover, no 'oomph' and engine running in fits and starts. I dropped it off with my local dealer who ran the gamut of diagnostics on it and a faulty lambda sensor was diagnosed and this has been replaced. The engine has regained it's 'oomph' but the tickover is still variable (somewhere between 500 and 1500rpm) and my dealer is at a loss to explain this. The ECU has been reloaded with the latest release of software and 'retrained' but it doesn't make a difference.
Any thoughts from anyone on the cause?
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