To get back to the stability you had from 10.5.5 or 10.5.4, you might have to backup, and do a clean system re-install. One might be able to find the offending .plist, preference pane or other file(s) to delete/replace. But unless you knew exactly where to look, a clean re-install of the OS would still be faster to get back to smooth sailing.
This happened for many people during the move from Tiger to Leopard as well (10.4 to 10.5) but was to be somewhat expected. It has also happened during minor system updates also but with much less frequency.
I’ve been advising people who do a lot of audio (some for a living) not to allow system updates until the smoke has cleared on each. (turn Software Update control panel off)
If they have older Macs (2-4 years old) and if they choose to use older apps (by older I mean circa 2005-2007!) or plug-ins, then I advise them away from Leopard (10.5.x) all together unless an app or plug requires it.
On my ancient G5 (10.4.11, RME9632), Renoise 2.0 has never even come close to crashing and it runs more AudioUnits plug-ins simultaneously than Digital Performer and Logic did. I am astounded at how well Renoise 2 handles AU plug-ins and CPU efficiency on such an old computer!
Be sure to keep your main folder(s) where you store you most important compositions backed up (if not your entire hard drive) as often as possible to preserve your sanity. I know this is a pain so I often set up automated backups for people to run every day to a dedicated external hard drive. Intego’s Personal Backup does this quite well and is non-intrusive (even while you’re working).
Beware Apple’s Time Machine! This backup program saves your data in its own proprietary format. If your Mac crashes, you will need a working Mac to restore your data. Also, shortly after the release of Time Machine, there was talk of a class action suit against Apple due to corruption of customer’s data saved in the Time Machine format. Fortunately Apple solved that particular issue but I never trust backup programs that save data in their own proprietary format like Retrospect, Time Machine and a few others.
Finally, don’t forget to save the installers of your favorite pro audio plug-ins and applications to DVD-R discs. Since many of our favorite programs originate from download purchases, we (I) forget to put them on a separate disc just in case of such emergencies as having to re-install the OS.