AC cycleing

Hello.
On my 2000 when I turn on the defroster the idle fluctuates up and down but I can not hear the ac cluth cycle. This happened last year and only required four oz of freon and then it still cycled a bit.
It is not a wild idle swing but it makes the truck lurch a little at idle and surges when cruising at steady speed.
Any help will be appreciated in this matter.
Thanks in advance. Steve

Open the hood and see if the clutch is cycling.
The idle will come up a bit when the clutch kicks in, but it should not be that noticable.