There is a "dual pressure" switch in that system - it detects both high and low pressure and locks the compressor out.
My suspicion is that you now have too much refrigerant in the system and when the compressor cycles in, and the pressure builds, it trips the high pressure switch, the compressor cuts out and the pressure drops allowing the switch to close and the compressor to kick in again. Part of this suspicion comes from the fact that it will run for several minutes when the system is first turned on, the internal pressure does increase as the system warms up.
Find the switch (it should be near the receiver dryer) and wire a 12V test lamp across it - if the switch is closed (as it should be) the lamp will stay off, so if you see the lamp flashing on when the compressor goes off, you know you've found the cause.