Up and Down Idle with A/C on?

The idle on my 2000 Expedition (4.3Ltr) surges up and down between 400 and 1100 rpms when the A/C compressor is engaged and the vehicle is in gear. Turn off the A/C and the problem is gone. Perfect steady idle. No check engine light or any other problem at all. 58,000 miles and I changed the plugs less than a year ago. Is it possible that the compressor is dragging the engine down? Or is it more likely that this is some kind idle regulation problem? (TPS, vacuum line leaking, that sort of thing?) If anyone has a clue what might cause this let me know. I'd appreciate any information at all.
Thanks!