I understand what you are telling. Yes, the pump centre section was removed to replace the oil seals as it was leaking.
It was done very carefully by myself with proper markings for aligning. After replacing, the engine started first time and stayed at 750RPM idle without any fluctuations whatsoever( I was extremely lucky). All that was more than two(02) years ago. It ran perfectly until December 2016, when I replaced the HG,Timing chains, Sump gasket etc. But the inj pump was not touched except for removal of sprockets & injector lines.
As you say if it's the centre section, then the bolts should have loosened with usage which I think is unlikely as there should be diesel leaking or else it got to be something like a worn bush in the shaft(don't know the exact name but it goes down in the middle of centre section, which gives accelerator motion).
With all that said, I guess that the pump needs re-calibration of sort. But the local Bosch agent does not appear to understand a VP37 in a BMW.