what would cause and engine oil pressure guage to be perfectly fine about 2000rpms but when you drop down below say 1800 or at idle it begins to flicker slightly.it will even do this if im doing 55 and push in the clutch and the rpms drop.it does have oil in it.ive heard on fords there is a oil pressure sensor that does go out sometimes.

 

You just answered your own question. That would be a place to start atleast. Past that I am thinking oil pump?