It sounds like you covered all the bases, even down to having the governor springs replaced so itís hard to suggest any other things to try.... but maybe this will help!
On the skinny metal linkage that connects the fuel
rack to the governor there is a small square of metal which is the method of pushing the control link into the stop position by using the manual stop lever. If this little tab is bent by over enthusiastic use of the manual stop lever, the unresponsive rapid and unpredictable idle may/ will occur. Hereís a pic of the stop lever and the tab that I posted previously on the forum and it might be worth taking a look through the oil
fill hole on the front cover near the stop lever, the tab should be about 90į to the linkage but is sometimes a bit over the 90.
What were the circumstances leading up to the erratic idle if any?