Not sure it this will help, but I had a similar problem with my Audi. It ended up that it was the fuel pressure regulator. see http://www.sjmautotechnik.com/ficis.html#mixture
I usually had the problem after the car warmed up.
'98 E320 90k
'91 Miata Turbo 280hp
'90 Miata Turbo 230hp
'90 Audi 200 turbo 170k