Ive heard of carbs making the engine idle up then down etc.....kind of what your saying yours is doing. But Ive only seen this happen on carbs. that were not feed back carbs. (not saying it wont happen on them). I had a 1974 ford maverick that did it and my dad and couldnt figure out what the problem was. We rebuilt the carb., we checked out everthing under the hood all the emissions stuff and it still did idle up then down. Finally we brought it to a machanice and they said it need a new carb. After that ran 120 mph and idle great. But it wasnt a feed back carb. My dad has a 1984 ford ranger 2.8 liter
4x4. When we got it, it was just cold blooded. It wouldnt start easily, run for s!@#. So we rebuilt the carb. Still had the same problems. We went through all the emissions stuff and they were fine. Finally we put a used carb on it and to this day havent had a problem and it run great. My piont is from what ive read and my personal experances, it sounds like the carb. might be the problem. If you had a video of the truck idling, running, the clicking sound, it might make it easier for people on here to help you out.