My 900S has started to stall when pulling up at lights and when driving "off the throttle". I can also reproduce this behaviour by starting the car from cold and leaving it running at idle for 20 minutes!
My car has done this a few times (less than 10 times in 22-k miles). Most times the A/C was on, and I was turning and applying the brakes. So it might be a combination of additional load on the engine by the power steering and air conditioning compressor in addition to a reduction of vacuum in the intake because vacuum was being applied to the brake booster. Just a guess though... Ron