After wrestling with the 8.6 update for days, I finally got the update to take and not crash my system at intermittent times. After reading many postings, I performed the following: trashed finder prefs, trashed ATM Deluxe 4.03 prefs and sets, zapped PRAM before and after the install, rebuilt desktop. This was done after attempting to install 8.6 "ten ways from Sunday" "Dirty install, Clean install from 8.5 to 8.6, clean install from 8.5.1 to 8.6, install from 8.5 all and 8.5 base, etc. I tried it all.
I am still having one weird behavior. About every 10-15 minutes, my G3/300 beige minitower pauses (I can move my mouse) but there is no Finder functions, after 10 seconds or so, the system picks up right where I left off. If I am in the middle of typing something and the system pauses, when it releases the system, the words fill in to the point where I stopped after the pause. I have turned off file sharing and trashed the prefs. I have allocated more memory to everything I can think of. Does anyone have a clue?