engine reving with A/C

I have a 1991 with a 4cyl 22Re, 150K miles. Last summer the A/C light on the button stopped lighting up when pushed so I thought maybe the A/C needed serviced. Had it checked and charged (it was down just a little). At about the same time I noticed that the engine will rev up and down when at idle and the A/C is on. It will rev about 500 rpms for a few seconds then back down then up and back; it will do this for a long time. The A/C works fine, just no light on the button and the revving. Any suggestions? Thanks H-S-F