its normal for the pressure to increase when you rev the engine, since the pump works harder, but on idle the guage must not read 0, if it does and the guage and the sensors are ok, this is a sign that the engine bearings, or camshaft bushings have a lot of wear in them making the oil pump loose pressure, or maybe the oil pump is also worn out. or try to check the oil strainer maybe its just clogged that the oil pump cannot suck in oil as needed. one remedy is to try to change the viscosity of the oil your are using to higher ones so that the gaps may be filled with thicker oil, or add mo oil thickeners like mga STP, Bardahl 2.