Premiere Pro :: Mercury Playback Engine GPU Not Available

I have a 27" iMac (late 2012) with NVIDIA GeForce GTX 680MX graphics card running Mac OS 10.8.5.
 
I'm using Adobe Premiere Pro CS6.0.3, and when I updated my OS to 10.9 Maverick, I began to experience several problems, including:
 
The dreaded spinning beachball and "Premiere not responding." "This project was last used with Mercury Playback Engine GPU Acceleration (CUDA) which is not available on your system. Mercury Playback Engine Software Only will be used.Video files - I think they were all .MOV - became "squished" to the top of the Program Monitor (movie only took up about 1/3 of program monitor window). After a long chat with Adobe Support, he indicated that the update to 10.9 Maverick was the problem, so I:
 
Updated to Premiere 6.0.5, but that did not fix any of the issues.Logged in as "Root," which worked for a while but I also received 3 "Premiere encountered problem and must shut down."Deleted preferences, repaired permissions, basic troubleshooting practices, etc, but problems persisted. 
 
Eventually I restored my computer from a Time Machine backup to a day prior to the OS 10.9 update, and this seemed to fix all the issues. So I was back to Mac 10.8.5 and Premiere 6.0.3.
 
But the .MOV-compressed-in-the-program-monitor problem began to surface again (see  photo below).
  
So I updated to Premiere to 6.0.5 (but not my OS - it's still 10.8.5), and the "Mercury Playback Engine" error is back.
  
And the .MOV files are still compressed in the Program Monitor. I have a project that I completed in 6.0.3, but is now messed up in 6.0.5.
 
Is the only solution downgrading to Premiere 6.0.3, and if so, must I do a clean install of the program from the DVD?