A/C is erratic, blows cold sometimes and warm at other times

It is summer and when we turn on the A/C in the vehicle, we set it to the lowest temperature but it blows warm air. Sometimes, out of just sheer luck, it'll blow very cold air and sustain it until you shutdown the engine. At other times, it will start blowing warm, then without warning, start blowing cold. This is so erratic and unpredictable that we just leave the temperature setting on the lowest, keep our fingers crossed, and hope that it starts blowing cold or switches over to cold air during our trips.