Assuming your system is full, the problem could be either a loe pressure switch that is cutting off at too high a pressure or a loss of hysteresis in the control loop. Pull the connector off the low pressure switch and short the connector terminals together with a paper clip to see if the problem resolves. If yes, verify that the low system pressure is up to spec before having the switch replaced (system must be evacuated to replace the switch). If shorting the switch has no effect, the control system is likely the problem. I would need to know your model and year to tell you where the control circuitry is located.