Volvo Repair: 2003 S40 T4 intermittent idle problem with loss of turbo boost, air control valve, boost car

Question
James - I recently purchased a 2003 S40 T4 manual with about 43k miles. Runs well most of the time with steady idle at 750 rpm and good power. Intermittently idle becomes irregular oscillating steadily between about 500 and 1000 rpm, sounds like the motor wants to stall. When this happens there appears to be no turbo boost, car is still driveable but does not have turbo kicking in as it normally does.
No warning lights displayed. Condition can always be cleared by simply stopping and restarting the motor, then runs OK and may do the same again after some time. This happens maybe a few times in a week. Car is in daily use for commuting.
Many thanks for your advice,
Regards,
Clive

Answer
Well those cars commonly have problems with the idle air control valve, which would be the first thing to check.  Your problem would be hard to pinpoint since it is so intermittent.  I suggest you take it to a dealer or shop to get a read out from the computer because chances are it has posted a DTC.  With that information usually the problem is obvious.  I can help with the replacement of components but you're going to need more hands on for the diagnosis.

James