I'd change the rod/main bearings, go with a new sealed power std vol/psi pump, make sure the pickup ain't too far off of the pan bottom, use a non Fram filter. since you are torn down I'd rering with some plasma moly file fit rings & dingleberry hone the cyls lightly with their 320 grit flexhone. That far down you could almost bore for new pistons to get the SCR up but a person has to draw the line somewhere. You said the pickup is clean & the NPT plug at the rear of the DR horizontle gallery INSIDE the valley is in place. EDIT to ans the orig Q I'd guess a combo of worn bearings and or partly clogged filter/worn pump assuming the gallery plug is in/pickup distance correct/pickup not partly plugged