DIAGNOSIS That rapid cycling is usually the unit hitting its internal high-pressure limit and shutting the compressor down as a safety measure. The most common cause is a dirty or blocked condenser coil (the warm coil on the back).
FIX Unplug the unit and pull it away from the wall. The condenser coil is on the backside, behind a metal grille. Use a vacuum cleaner with a brush attachment to gently but thoroughly remove all the dust, lint, and pet hair clogging the fins. Make sure there's at least 12 inches of clear space all around the unit for airflow, especially the back.
IF THAT DIDN'T WORK If cleaning doesn't help, the ambient room temperature might be too high (above 90°F) for it to shed heat properly, causing it to over-pressure. Moving it to a slightly cooler location can sometimes stop the cycling.
HEADS UP This cycling is hard on the compressor. Letting the coil stay clogged is a sure way to shorten the unit's lifespan. A quick vacuum every couple of months during heavy use makes a big difference.