Sounds to me like it could be either a failing gauge, a loose wire, a faulty sensor, or a faulty sending unit if the car has plenty of power and is running fine otherwise and does have proper pressure.
You should have it checked out to verify it is not an oil pressure problem, you don't want to drive without any oil pressure.
When you are at idle, it should be reading around 2 I think, when you accellerate it should go to 3 and stay there while you are driving. but at a stop it is normal for the gauge to drop a little bit.
I'm not a tech, but from what I have read on this site over the last several months, it sounds like one of those items could be it.
'92 300CE - Sold
2004 C240 - C7 Wheels
2015 ML350 - P1, Pano, Ash poplar wood, Sport Dinamica interior, Running Boards Keyless go.