From experience, I've found 8.1 to be quite a nuisance on my 5215 since the OS tends to freeze for random reasons. For instance, if I keep the unit on Sleep mode for longer than 3 hours, the system hangs upon waking up. When my parents still had an AOL account, the system would freeze if it could not connect to AOL (or would suddenly freeze when trying to access e-mail, etc.). Also, when I transfer pics from my QT 200 to my external Avid HD, the system says an error occurred, and hangs.
Personally, I think your problems are with the OS. I'm not sure if 8.5 is better in terms of stability in comparison to 8.1, but I'd like to give it a try on my 5215 if I ever find a copy of it.