Very tough situation. Do you have access to a scanner with actual data? If so you will want to see freeze frame data and see O2 sensor reaction times. The long term fuel trim should be evaluated to see if the problem is intermittant or always borderline. In other words if the car runs at high fuel trim the this is an area to pursue. If it looks low during evaluation then an intermittant can be the problem.
Common failures are the airflow meter, oxygen sensors, and possibly fuel supply issues (filter, pressure regulator, plugged injectors).
One interesting thing I have found is that the scanners that pattern the factory scanner leave out many of the OBDII protocols. I have pulled pending codes off OBDII cars with my MasterTech scanner in Generic OBDII that weren't apparent with the AST retreiver or CS2000. The MasterTech or EASE PC program will calculate O2 sensor reaction times also.
Bosch Master, ASE Master, L1
33 years MB technician