crafty, Graham:

In addition to the idle air controls, it seems in order to suggest a bit of attention to the idle fuel control. The pulse width to the injectors is adjustable at idle by the mixture screw on the ECU.

Of equal importance is the voltage available at the injectors, which is a function of the condition of all the connectors and ground terminals. When the engine is at a low idle the output voltage of the charging system will drop, and when combined with accumulated corrosion on connectors and grounds can result in such a small signal to the injectors (already very short at idle), that an insufficient amount of fuel is injected to maintain idle speed under the load of an in-gear situation. In Neutral the load is removed, idle speed increases slightly, voltage rises, and injector pulse increases.

Additionally, fuel pressure should be at or slightly above design specification.