Why is my lock screen scrambled sometimes?
My best guess would be an issue that is causing part of the render process of slide lock to take place in landscape mode. I have no idea why that would happen although I have had some strange screen rotation issues with my nexus 7 and a factory reset did indeed fix it.

But since you don't want to turn to that here are some suggestions:

-Try locking rotation on your phone (unlocking when needed)

-Switch your lock screen to PIN or password (there error could be isolated to the slide password).

-Upgrade to a new version of android using kies (it should be available)

Let me know if any of those work out