Found this thread while trying to find answer about the exact same issue with my 01 Zetec Focus SE .
A/C no A/C is doesn't affect the issue but it is random and when it does the trouble , I get no DTC either but if I reset the battery the problem goes away for days if now week.
I don't want to change parts just for fun and the Original poster did change a bunch of things and he's having a similar issue.
In my case I'll have rough idle for a few days and after "0" idle.
I rest the battery and gone .
Any tips where to look at ?