If your portable air conditioner is turning off and on every few minutes, your unit is probably too large for your room and is "short cycling."
This happens when cold air from an oversized unit bounces off the wall and back towards the unit, creating a room temperature reading that's cooler than the actual temperature. This false reading causes the compressor to shut off before the room has truly reached your desired set-point. While the compressor is off, the "real" room air enters the intake grille, and the thermostat reads that the room is no longer at the set temperature, so it turns the compressor back on.
Your unit cycles off and on approximately every two to three minutes to reach the set-point. While the air coming out of the unit is cold, the compressor does not remain on long enough to cool the entire room. The only solution is to correctly size the unit's capacity to the room size and demand.