[pidim]

Hi all,  would anyone have some ideas about this?
In recent months I have noticed that when my car is fully warmed up and I have been driving for a while and then decide to turn on the aircon, if I am stopped and in neutral, the engine struggles and revs lower, it then picks up again at normal RPM, only to do it again.
When I am driving and stop and traffic lights for a while (eg peak hour), it also stutters again and on one occasion the engine stalled and I had to re-start. 
The aircon itself is fine with icy cold air, but seems like some extra load or something makes it struggle.  Could it be a drive belt, IAC valve or a bigger issue?
This never happens when the engine is still cold and I start first up.  Only happens... maybe 30 minutes into driving.
Hope you can provide some ideas or solution.
Thank you.