I had the exact symptom in a 3.5. This thread from Arthur Dalton clears things up. Well, the guidance does, anyhow. The BOSCH manuals call the swirl pot the "header tank." The header tank inlet was being starved but the after sitting for several hours, the header tank would refill itself and the car would run fine - until the header tank was drawn down.

This was a few years ago for me but Arthur's post is the best gouge on the topic.

http://www.peachparts.com/shopforum/showthread.php?t=198008&highlight=swirl