Can spotlight be reset when it misbehaves, is wrong or constantly indexing?
Close all applications and stop Spotlight

sudo mdutil -i off /
sudo mdutil -E /
sudo rm -Rf /var/folders/* 
sudo rm -rf /.Spotlight-V100/*


then reboot and turn Spotlight back on

sudo mdutil -i on / 


These commands turn Spotlight indexing off, erase the existing index files and turn spotlight indexing back on.



UPDATE:

According to your latest information the above does not help in long term.

Lets try finding what makes it index.

Disable all areas that you "normally" do no search.



Add Folders like System to the do not search.