low revs almost stalling

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hi my Volvo s8o year 2000 its a automatic and is playing up . its started a few months ago its got quite allot of mileage on but an excellent reliable car. when I'm excelerating its fine its when i take my foot off the gas and is idling the tick over drops to about 400 revs and almost stalls this lasts for a few seconds and has occasionally stalled but usually picks back up to about 800 revs ... any idea what the problem is thanks Richard harmer UK