i'd listen carefully for any hissing noises, particularly around the inlet manifold at the back of the engine.

next step would be to clean and lubricate idle control valve, it seems to be a common problem (it wasn't which caused mine, but no 2 engines are the same )

if that doesn't sort it, get your ecu scanned by a local garage - could be any1 of many sensors.

if that doesn't solve it - check the inlet manifold for leaks - means taking it off, inspecting very carefully, lots of holding up to the light, etc, replacing gasket etc, and they shouldn't normally fracture (potentially a big job - mine took me best part of a day!)