Same thing with the DISA flaps for me (R5k gamble)...
I think in both in your and my case we need to have the fuel pump pressure tested and in my case have the injectors tested too to ensure that their individual spray patterns are up to spec. These cars don't have replaceable fuel filters (which is so stupid), so there is a big chance that they could be slightly dirty or blocked. It could even just be that the pump is weaker than it should be or that the regulator is not working properly.
Now just need to find a place that can test these things without immediately wanting to rip a person off. ART won't see me again - over 2 weeks later after leaving a very polite message to PLEASE call me back, and still nothing!
BMW E90 330i (M) '05 (His) | (Hers) BMW E46 320i f/l (M) '03