The car in question is a 2001 S Type with 3.0 liters engine.

The symptoms are just like a vacuum leak, according to this forum, would cause. However, these symptoms *only* occur when A/C is on. Without A/C the engine runs fine.
No fault codes.
So....which vacuum line or part to check?

Also: Does this have to do with the problem? : When I try to cool down the car with A/C on, the passenger side gets *much* warmer air for at least 5 minutes, than the drivers side. No matter if DUAL-position is selected or not. And I think the A/C actually never blows as cool air as it should.
When the heater is blowing hot air, there is no difference between sides. Or problems with acceleration or/and idle.