Okay, bit of a guess but I might as well relate my experience and if it matches yours, then here's how i solved it.
In my case I noticed it only came on during long periods (about two minutes or so) of decelleration, for example on a long hill. Or ticking over at traffic lights. If I got on the motorway immediately after starting and cruised, the lambda didn't come on. Once on, it didn't go off again no matter what style of driving. And the only way to clear it was to sit with the ignition off for about five minutes.
If your symptoms EXACTLY match these, then it's the throttle position sensor. Like a dirty volume knob on an old radio, the carbon track wears at common points (e.g. tick-over) so gives a false signal. Replacement is the only option, but you can get a cheapie from ebay - they're fitted to dozens of cars by a plethora of manufacturers. This is one case where paying the volvo price is unnecessary.
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