have you recharged it recently. there's multiple sensors in the A/C system, any of those can cause the clutch to disengage. if you charged it recently, you may have overcharged the system, causing the system to build pressure for a few seconds, then kick off and let pressure bleed from the high to low side, then kick back on, etc. if it's running for minutes at a time, it's likely just what awptickes said.
is it blowing cold? how long does it stay engaged for? is the belt that runs the compressor in good condition?