When I stop at a junction or lights after hardish driving the idle speed fluctuates between 200 and 1200 about 5 times before settling at 750rpm. After normal/town driving the idle is always perfect. My Subaru main dealer looked at the problem and plugged the Select Monitor into the car but found nothing wrong.

I've tried cleaning the MAF and idle control valve with carb cleaner (as suggested in previous threads) but the problem is no better. Possibly the Lambda but fuel consumption is no worse than normal.

Any ideas (UK MY99 turbo)?

 

The MAF.
It doesn't need to be showing an error to be reading wrong.

 

If the MAF is at fault wouldn't the idle speed be problem all the time?

 

Check the lambda sensor, MAF and ISCV.

 

mine is fine when car is warm but from cold car starts fine but as soon as i come to lights or rbout i put clutch in and bingo 4-500 rpm then back to normal.have replaced idle valve after i ballsed up the orig(ps dont do this unless u sell hard drugs and have access to large amounts of sedative).someone has posted thay had same prob and it was 02 sensor.john my petrol consumption is a bit down wld 02 sensor be worse when cold???

 

Change the "maf" sensor, its the most common failure item with these symptoms.

 

Check also the TPS (Throttle position sensor). If it has a bad offset, it can produce such variations. It already happened on my Forester and it was the same behavior than yours.