ERRATIC IDLING

I think the only thing to do is find a Ford/Mercury specialist. There are too many things that could cause problems to guess. If you want to see a specialist, here's a directory link for you: https://repairpal.com/mercury-repair-in-hillsville-virginia

Answers in order: Yes, the PCM is a known issue with that vehicle. Yes, obtain a "remanufactured" PCM, as that is all you can get these days. No, you cannoot use a programmer -- this is why you have to replace it in the first place, because the PCM's in this model is not programmable. When the software was faulty back then, you had to replace the whole thing.