1999 Honda CRV AC compressor turning on/off too fast

Please help! I am having trouble with my ac compressor. When it is in idle mood, the ac (clutch) turns off/on too frequently. I bought an amatuer gauge from autozone and it read about 20 psi barely passing the blue mark, so I added freon until it reached 40 psi which falls in between the recommended gap per the gauge with respect to the outside temperature. The air is cool but it still does it.I noticed when I rev the engine and let it drop to idle speed(about 500 psi) the compressor does this until I guess the pressure equalize. I know this is not normal, does anyone have idea? In addition I have read threads online. So I checked the pressure switch. It has 5 volts on the red/blue input wire and the switch has continuity.