Oil Pressure Drops -91-300E

[91-300E]

The OIL pressure in my 91-300E is at 3 when the car is cold and it runs at 1500 rpm. Once it warms up and it idles at around 700 rpm my oil pressure drops to ZERO.
A mechanical gauge measured ZERO at the Oil sender when the car is ideling.
Just changed the OIL pump (very expensive) New timing belt tensioner and I still have the same problem.

At accelaration beyond 1000rpm the OIL pressure is 3.
I am worry about the ZERO reading at idle.

Please any one help
Thank you