I have the exact same problem. After a period of work, it just seizes up forcing me to lock screen and then just turning off the screen. Again it's most usually caused with the pull down tiles. A thing I noticed is when it seizes up you have some time before it locks you out. At that time I pressed the home button on the touchpad and it took me to the main screen. At that point the pull down tiles and the bottom 3 buttons (back, home, tasks) are unresponsive. I've now reset the Davik cache and cache, will see if this fixes the problem.
Edit: After the reset same thing is occurring, but this time there is no lockout. Just the menus becoming unresponsive. If I press one of the menu buttons everything becomes unresponsive, after a while it return to normal (except the menu buttons). If I press them again ... well just repeat the above. After a couple of times the tablet shuts down.
***I used the toolbox with pa_app, Cyanogenmode11 (the one from Jcssnuell (or something)) and Clockworkmod - all after deletion of previous systems. Before that I had Cyan9 which had the same issue i believe.