I have the exact same problem. I have a Galaxy Ace and from time to time instead of the assigned ringtone it plays the standard ringtone. This happens at least 2-3 times a day. The sms tome works fine but the ringtone messes up. My wife has a Galaxy Gio and she has the same problem. Both phones run android 2.3.3. We updated from 2.2 and we didn't have this problem.
I have googled this a lot and didn't get an answer.
Do you have a solution yet?