As for the Actions, what are the Playback Options?
Have you properly restored the Preferences (ctr-alt-shift on starting and the dialog "Delete the Adobe Photoshop Settings File?") or manually deleted the files?
Please read these:
I've been using the same actions for close to 4 years. It just started happening all the sudden.
I did the ctrl+alt+shift as well as manually deleted the files.
What are the Playback Options? Edit: See the Actions Panel’s fly-out menu
They are set to Accelerated. It's always been the same. Things started going wrong after I did a disk defrag. I'm not sure what happened. When my actions prompt me place an object, it no longer shows a thumbnail either.