There are a few a few safety devices on the AC that could be doing it. Low evaporator pressure (vacuum), and high condenser pressure. The low evap would be from a low freon charge. The high condenser (head) pressure could be from over charge, (liquid freon getting into the high pressure section). or a dirty condenser coil, or low air flow over it. It is the coil in front of the radiator. Try spraying a mist of water over the condenser coil to see if it keeps running. If so, it is high head pressure doing it.