As described in the just previous post, FCP7 has a ugly playback defect under OSX Mountain Lion which it does not have under OSX Snow Leopard. Independently, I had a controller card that was working poorly with OSX Snow Leopard and well with OSX Mountain Lion. So I tried OSX Lion, in between, for the one-quarter chance it would solve both problems. Hurrah!
Now I've tested FCP7 under Snow Leopard (10.6.8), Lion (10.7.5), Mountain Lion (10.8.5), and Mavericks (10.9.5). Each was a fresh FCS3 install into a freshly installed operating system. The FCP7 playback defect starts with Mountain Lion.
I saw the FCP7 playback defect with a fast edited 1920x1080 50p ProRes422. There is occasional breakup at horizontal lines on the 100% scaled Canvas. It looks like incomplete screen refreshes. FCP7 playback has this defect, while QuickTime doesn't.
I've uploaded 13 seconds of the acid clip --
here -- so others running FCP7 under Snow Leopard, Lion, Mountain Lion, or Mavericks can help confirm that the playback problem begins with Mountain Lion.
Dennis Couzin
Berlin, Germany
[Note added 4 January 2016: any 1920x1080 50p ProRes422 will display the same playback symptom as the "acid clip".]