99 ram diesel oil pressure drops

[DMaclaren80]

My 99 ram 2500 diesel the pressure is high when I start it then comes to normal when driving. But after five to ten minutes at 70mph on freeway when i stop on the off ramp it drops to almost nothing. the dip stick says its in the safe zone. any ideas on whats happening please help

 

[Merc225hp]

The gas motors oil psi sending units in these trucks suck, they fail lots. Not to sure if this carries onto the DSL motors. Remove the oil psi sending unit and install a mech psi gauge see what you get for psi using that.