What exact version of ACR? If it's not at least ACR 7.4, you need to update the ACR plug-in.
Also, you need to update Photoshop ASAP. We're at 13.0.5 now (Mac):
Repair Permissions with Apple's Disk Utility after applying the updates.
If the issue persists after you have done all that, you may need to trash and reset your Adobe Camera Raw Ptreferences file at Users/[your user name]/Library/Preferences and relaunch the application.
Message was edited by: station_two
Please note I've edited my last post to add "with Apple's Disk Utility".
Edits to posts are not broadcast by e-mail.
This has been a recurring problem, at least since I updated to CS6 last year, and possibly also with ACR in CS5 (but I'm not sure about that). It usually is gone next time I open ACR, and is unpredictble. It can be embarassing in class when demonstrating tonal or color values while teaching RAW. That is not a good time to trash and reset preferences.
However, next time it happens, I shall try exiting out of ACR, then relaunching.
No, I haven't updated to 13.0.5 yet; I usually wait at least a week before updating.
But, thanks for letting me know it wasn't something I was doing wrong.
Obviously you have problems with your machine. What you describe is not normal and I have not heard about anyone else experiencing it.
It could be a memory problem, whether bad or mismatched RAM, or RAM going bad or simply improperly seated RAM. It could also be a hard drive issue, or a CPU or GPU issue. Intermittent problems are the toughest to troubleshoot. It could also npoint to an overheating problem.
How much RAM do you have installed and how much available space do you have on your boot drive?