Cummins M11 oil pressure issue

[lhwood]

Looking for ideas here.  I have a '99 M-11 Celect Plus that has an erratic oil pressure issue at 1500 and higher RPM's. It makes normal pressure and then the needle dips and dances if held at a steady rpm.  This is observed on the CELECT as well as a mechanical gauge.  I've changed the pressure regulation spring and verified that the related system is correct per Cummins specs.

Thanks in advance.
Durwin