Q: My car has idling problems that come and go

Hi, Recently I have been having troubles with my car, I have spoken to the local mechanics about this and they are honestly quite useless and didn't seem to have any idea. Anyway, my car has a 2.4 litre Mitsubishi petrol engine in it. - 90% of the time it will start and run fine however the idle is never consistent. - The other 10% the engine will either start and then die immediately, or will surge anywhere from 250-500 rpm. Pressing down on the accelerator will keep it going however as soon as I take my foot off the engine will stop again.

I would like to know what could cause this. I have replaced spark plugs so I don't think it would be that. I have also cleaned the throttle body out.

It has also been suggested that a vacuum leak Could cause this. There is a high pitched whistling noise coming from the engine but I cannot find it.

A faulty o2 sensor has also been suggested. But really I am at a loss to explain.

Any help would be much appreciated.