The low pressure switch causes the compressor to shut off around 20 psi, which makes the pressure go back up (due to no A/C function), then as it climbs back up, the compressor turns on again, only to run low on pressure, shutting the compressor off at 20 psi, and the whole cycle starts again. It's probably low freon. I had the same symptoms, and I added a can of freon, and the A/C system now holds at around 28 psi. The idea is that the freon is so low that it would run even lower than 20 psi, but the low-pressure switch shuts off the system to avoid compressor damage due to low freon. Which is why cars missing a LOT of freon won't even turn on the compressor at all (because it's already below 20 psi). The static reading for low-side with the car off is often 60-85 psi.