This is similar in a way to the problem my VX had and took the dealer 2+ weeks to figure out.

In the mornings when cold the car would NOT hold idle. Turn her over and drops from 800 to 0 and dies. After 4-5 times starting the car would sit there and hold idle at would I kind of figure is around 300 RPM. (not enough to do anything). Shut her off and back on 2-3 more times never having moved and WHAM jumps to 2000 rpm and sits there. Drive it for a block and then it is fine. The problem then stops for 2-3 weeks. Then this last Friday in stop and go traffic it did it on the freeway (that really sucked). Took it to the dealer and they ended up replacing the idle control sensor, some intake gasket, and about 6 other things. (which I think were just replaced to take on to the labor they get paid as it was under warranty still).