idling irregularly

[david75]

Rover 75 2.5 v6 petrol auto

When coming to a standstill after a run, put in Park or Neutral, normally engine should idle steadily at 650 rpm or so.

I've noticed that mine sometimes seems not to idle steadily, but to gently rise and fall between about 600 and 1200 rpm, each rise/fall cycle taking about 1-2 seconds

No other obvious symptoms or ill-effects. No problem driving off.

Problem seems to go away and idle ok if I turn engine off and on again.

Only seems to happen when engine is warm after a run, and even then not every time; and not when starting from cold

Any ideas please.

thank you. David