DIAGNOSIS This is usually the thermostat doing its job, but poorly. If the thermostat sensor is in a bad spot (like right next to the cold air vent) or the room is very small, it cools that spot down too fast, shuts off, then turns back on when the rest of the room is still warm.
FIX First, make sure you're not in "Eco" mode, which is designed to cycle more aggressively to save energy. Switch to a standard Cool mode. Second, check the temperature setting. If it's set too close to the actual room temperature (like 75° in a 76° room), it will short-cycle. Try setting it 3-5 degrees lower than the room temp.
IF THAT DIDN'T WORK The exhaust hose might be letting hot air leak back into the room near the unit's intake, fooling the thermostat into thinking the room is heating up again instantly. Double-check all seals on your window installation kit.
HEADS UP Short cycling is hard on the compressor. If it's happening every 60-90 seconds continuously, it's a problem. If it's every 10-15 minutes, that's relatively normal operation for a portable AC in a well-insulated space.