2006 Sprinter Fuel Problem HELP

2006 sprinter model 2500.

No power, will start but not pull. Fuel pump continues to run, loud hum.

Replaced fuel filter, no improvement.

Fuel pump was replaced a year ago.

Per the manual, removed fuel line from filter and placed a guage in the fuel line, reads 120psi and fuel pump stops humming.
Placed a "T" in the line into the filter, pressure was 35psi and pump runs (hums).
Started engine, pressure ranges from 30 to 35, engine either stumbles or even stops.

Manual says replace Fuel Pressure Regulator on bottom (lo pressure side) of high pressure fuel pump. Obtained replacement Mercedes Part from EUROPARTS of SD, installed, BUT no improvement!!!!!

(Installation was painfull) Results moreso!

What have I missed?????