Problem solved!
I contacted Wacom tech support by email, and then later by phone. They sent email instructions, which I've copied below. That didn't work, but then I was told to try this: in Photoshop preferences, go to the Performance pane. There is a box for "Enable OpenGL Drawing." UNCLICK that box. That's all it took. Apparently, it is a Mac OS issue that they are now aware of and should be fixed in an upcoming update. I don't know if this will also work for PC users.
Here are the suggestions from the tech support email:
Depending on your version of Photoshop, you may want to check out some of the suggestions from Adobe to optimize performance. Also, follow the steps at the end of this email to make sure you have the most up to date driver installed correctly.
CS4
http://kb2.adobe.com/cps/404/kb404440.html
CS3
http://kb2.adobe.com/cps/401/kb401089.html
You must be logged in as the administrator
Make sure your tablet USB is directly connected to your computer, not a hub or otherwise (ex: keyboard)
Close all open applications
Please, uninstall the current driver using the steps noted below:
Select the Go menu and Applications
Open the Pen Tablet/Wacom Tablet/Tablet folder and Remove Tablet (if more than one, remove all - if you have none, skip to Repair Permissions)
Click on Remove Tablet Software option at the bottom right of the pane.
Enter your password and click OK.
A confirmation of removal will display when complete.
*Repair Permissions by following these steps (make sure you have all of your software and OS updates done):
Select the Go Menu / Utilities / Disk Utility Select First Aid and the disk (commonly named: Macintosh HD) you wish to repair then click on Repair Disk Permissions
Enter your system password when indicated and the repair will begin.
This may take a few minutes depending on the size of the drive.
When complete close Disk Utility and restart your system. (if the restart hangs do not force the computer to shut down, it is rebuilding files that were damaged, corrupted or misplaced)
Install the 6.1.1-2 driver for Mac OS X 10.4 or higher, from our website, I have provided a direct link for you if needed:
http://www.wacom.com/productsupport/download.cfm?id=195&product=PTZ
Select energy saver in your system preferences you'll see an option to change your sleep preferences. Suggestion: please uncheck "put hard disk to sleep when possible".
Lastly, you shouldn't have this issue if you have ample hard disk space on your computer. However, to check your hard disk space, make sure your available memory is at least 30% of the capacity. You can highlight your Hard Drive on your desktop and hold down the Cmd key and press the letter "I". If the Available is less than 30% of the Capacity, your computer is running insufficiently.