If you "have no idea what", then why can't it be a fuel mixture problem? You don't tune "cold start" fuelling on a dyno. It appears to be a cold start fuelling problem.Is the boost gauge supposed to read anything less than 0. If not, then it will always read 0 at idle - boost doesn't generally happen until at least 2000 rpm, and then only when the car is being motivated. (It's possible to actually rev the engine to red line in neutral, and get no boost.) If it is supposed to read something less than 0, then it could be some sort of air leak, but I expect that an air leak that produced no vacuum in the manifold would prevent the engine from running at all.
It could be the cold start valve not opening up when cold. It's located under the throttle body on RB20DETs, and under the inlet manifold on RB25DETs.
Duncan, riceline,
Thanks for the translation
(oh, and it's "apostrophe")