Honda Civic air con compressor on and of frequently.

Hi I own a Honda Civic 1999 1.5 vtec 60,000 miles. When I use the air con the pump keeps switching on and off every several seconds and feels strange when driving at a constant speed. The power fluctuates as the compressor turns on.(It dose this even when the blower is set to max, the compressor is supposed to stay on when set to max).
I have owned the car for 3 years and it has only recently started to do this. The air con still cools the car. Any idea what the fault might be?
Thanks