i've noticed the same type of thing w/ 8.6 on a beige g3. i think it helps if you delete the index and let it re-index from scratch once in a while. (you can do that from within sherlock, under 'find by content' there's a button 'index volumes...' which shows all the mounted vols, when they wer indexed, etc. just choose the ones you want, and hit the 'delete index' button) i usually do that after in rebuild my desktop.
hope that helps,