TDI Engine: Jerky Performance & Soot

A friend's 1997 Audi A6 C4 2.5 TDI (AAT) has been exhibiting the following issue for the past few days:

When accelerating, the engine starts to jerk violently, accompanied by increased black smoke emissions. Additionally, the car sometimes loses power on the highway; it simply cuts out without warning, and then resumes normal operation after a few minutes.

I already had the error memory checked, but there are no error messages. I also replaced the diesel filter and inspected the EGR valve, the mass airflow sensor, and the catalytic converter, but I can't find anything wrong there either. I thought I might have found the problem with the injector control valve, as a cable was frayed there and probably only had partial contact. I've now resoldered that cable, so that should be ruled out as well...
Does anyone have any other recommendations for things I could watch?

Best regards,

Hannes.