Does sound like low refrigerant. This can slowly leak away over the years and will reach a level where ,as you describe ,there is still just enough gas left that the compressor will sometimes run, maybe for a minute, then shut down for a while before starting again . With poor A/C performance . But eventually the day will come when the level drops so low it no longer works at all.
As the car is nearly 20 years old a slow leakage is quite likely. If the leak is very slow its possible a regas would last a couple more years .. Its illegal in most countries for aircon specialists to recharge a system if they know it will leak away again quite quickly, such as recharging the system every summer or every few months rather than getting the leak fixed properly. . They should carry out a leak test by injecting a special florescent dye which is visible under ultra violet light if it leaks out.
If the leak is quite slow it may be possible to improve things by injecting a leak sealant into the system. Not all aircon specialists would be willing to use sealant as it could damage the system and is not guaranteed to work. But you can buy it. Might be worth a try on an older car.
But its possible you have had a sudden major leak such as a blown rubber seal, or stone damage to a condenser radiator. This could be expensive to fix and may not be cost effective on an older car.
You should also check the condition of the cabin pollen filter, which can affect A/C performance.
