I have not experienced it, but on the older S40 (up to 1999) there was a switch at the pedel to detect when engine management should regulate the idle speed. Around 1999 the switch was relocated to the engine compartment, but did the same thing. Possibly the later model still has this switch and is only out of adjustment. Sorry, can not help identifying or locating it for you, but this might put you on the right track. Or the switch can be permanently closed/open and the ECU can not control more than 400 RPM so when you open the throttle enough you override the idle speed controller.