I'm itching to find out the answer when it is discovered - and at present everything points to some sort of enrichment device (not "choke" - those only exist on carburettor engines). But it could be an electronic control (part of or attached to the Engine Control Unit - many have one or a set of resistors to control signal voltages which cowboys can tamper with to provide more fuel if they have turned up the wastegate on a turbo - a disgression in this case but a fault with such a resistor COULD be the problem) fault, or a sensor fault providing the wrong signal to the ECU, or it could be an associated device (like the "radio suppression relay, which is not actually a radio suppression relay, on 7 and 9 series Volvos) or it could be something like the ancillary injector rail pressure (on early V6 Volvos with mechanically controlled injection there were three different injector rails, one for cold start, one for normal running and one for in between).