Preview freezes DO

If MOV files are unstable my bet is they're being shown using QuickTime, though Opus's ActiveX plugin. (To confirm that, does it look like Apple's QuickTime's thin, dark grey seek-bar below the video? Or does it look like the toolbar/seek-bar you get when you play an AVI or WMV file?)

It seems to vary with different versions but QuickTime doesn't always work brilliantly and you should find things are better if you install the splitters/codecs required to play .MOV files via DirectShow (which is what Opus, Windows Media Player and most other things use).

If you do that then Opus's Movie plugin will be able to play the .MOV files and they won't drop through to the ActiveX plugin or QuickTime.

(You can also remove .MOV from the ActiveX plugin to prevent it ever handling them. To do that via Preferences -> Plugins -> Viewers, select the ActiveX plugin, click configure, then select Generic ActiveX at the bottom and remove the .mov extension.)

There's a guide to getting (most) .MOV files to play via the movie plugin here:

[HOW TO: Enable/fix playback of various media formats in Opus)