This is normal for the Ford system. They have a chart (I cannot find mine at the moment,) regarding cycle on vs off time. It was amazing to me the first time I saw it. In cooler (under 90's and higher) temps and at idling speeds, the compressor will draw the low side pressure down to about 26 or 27 lbs, and the low pressure switch will kick in, the pressure then builds up again and the system restarts. This can happen every 20 seconds or so at an idle. It's annoying I admit, like in winter with the defrost on and hearing that constant clicking because the air is running. Doesn't seem to hurt anything though.(In winter I disconnect the AC sometimes for days at a time, then reconnect it when the weather is drier.) I'd maybe recheck my charge level if you have a way to do that, a low charge can exacerbate the matter.
Happy exploring
Chris