You are not a rookie with diesels so I don't need to tell you that's part of their "charm".

My 1983 300D developed the same symptoms. I found I had worn out the accelerator linkage where it enters the engine compartment from the firewall. I wasn't fully engaging the injection pump. It's about a $50 part new. Fixed the problem. Mine came on gradually and I didn't realize how sluggish it was until I had trouble climbing a steep parking lot ramp from a stop. Replaced the part and it drives better than it has for years.