1999 Explorer--A/C problem?

Hello all.
My 1999 Explorer, at 275K miles, is starting to show its age. I noticed this evening that at IDLE (about 600 RPM) , the A/C compressor cycles on, then off, in a little less than a second. The clutch engages quickly andsolidly, but a bit noisily. Obviously there's no cooling at that rate. When I increase engine speed to 1000-1500 RPM, the compressor stays on, and the cooling is fine. My thermometer is sitting on my desk at work, so I can't tell you the vent temp.
At ~1500 RPM, low side pressure is 25 PSI, and the high side is 225. The system has never been opened.
Any suggestions?
Thanks,
Mike

My 1992 F150 did something similar to this, my mechanic just recharged the system with freon or what not and it has been fine ever since.