Its always best to start with basics and fuel pressure is most basic. Fuel pressure problems can be intermittant also.
After you have proven fuel pressure one way or the other, the next step is to identify what is changing. Throttle angle, timing, injection pulse width, integrator, actual mixture, O2 sensor value. Look to those things and find what is changing when it runs bad.
Bosch Master, ASE Master, L1
33 years MB technician