OK ... Im guessing that you have one of the following problems there ...
1. A corrupted Finder Prefs file. (Chuck it in the trash, reboot and a
new one will be created)
2. An extension conflict. Try booting with extensions off (Hold down the
Shift key when you boot) and see if its still unstable. If not ... most
likely you have either a conflict occurring between extensions that are
loaded, or a corrupted extension preference file or something like that.
3. A corrupted font file - eating up your memory. (Trash it if you can
find it ... ...