Since the fuel pressure has been checked, I assume it is right.
If the pressure is right (and steady), the mixture is adjustable. If it is adjusted and isn't right in other conditions, I would expect an airflap misadjustment or a differential pressure problem.
Differential pressure is the venue of the fuel dist so that shouldn't be a problem. With proper testing your problem should be easy. I would be back to the one who sold you the distributor. I would expect a better analysis for what you have paid for.
Bosch Master, ASE Master, L1
33 years MB technician