It is low on refrigerant, because it is low on refrigerant the compressor cycles on and off all of the time. Service the a/c and have the correct amount of refrigerant installed and it will work fine. You can get the stuff to shoot a pound of gas into it yourself or you can pay a shop to service it properly. Either way you need to understand that you have a very small leak in the system somewhere. This is evident because you are low on gas, i.e. it leaked out somewhere, and you still have enough gas in the system to allow the compressor to engage, i.e. a leak large enough to leak but small enough that all of the gas has yet to leak out. Make sense?

Mark