My Samsung ATIV Odyssey has been randomly locking up several times each day since upgrading to 8.1. Sometimes it unfreezes after about 30 seconds but usually requires a reboot.
Update: I won't be able to perform the hard reset until later tonight but on a whim what I did in the meantime was to go into Storage Sense and change "Store new apps" and "Store new downloads" from SD Card back to Phone. Why? I don't know. But so far, no freezes since I made that change. Coincidence?