Just a thought... I have followed this thread, for the most part, but can't claim to have read and soaked in every single post.
Having recently been going through my own dash and electrical connections and finding almost all of the pins loose in the pc boards and re-soldering them, have you verified that you have perfect connections at all those points? In other words, there are no places for potential voltage drop-offs in the system? Sure is starting to sound like that may be an issue with the fluctuating readings.
To me, (if all the wiring and connections are solid) it should at least be consistent, even if it consistently wrong.
