'77 450 Rough pulling at Low-Mid RPM

[Binford]

Hi All,
I have my Dad's 77 450SL (48,000 miles) for the winter to do a little work on it for him. It starts easily, hot or cold, and seems to run nicely with plenty of power once you get over 2500 RPM, but getting there it is quite rough and lacks power.
He had a shop replace several injectors a while back, but he said that made no improvement. Seems to be getting plenty of fuel - rich if anything.
Vacuum seems to be within specs, and spark looks consistant to all cylinders.
I have a good bit of experience with older cars (mostly MOPAR), but none with Mercedes. I do have the shop manual (CD) and Haynes book.

Lots of similar threads out there mention the CAT.
Thoughts?