Reduce Transparency Keeps Turning Itself Off

In the accessibility settings on Yosemite I have enabled the "Reduce Transparency" setting multiple times. The reason I am turning it off is to try to fix the issues causes with the menu redrawing when multiple monitors are connect and eventually slowing down the machine to a crawl. Each time I turn it on and go back in and check it has turned itself off.

It seems to turn itself off if the external monitor is connected.

Why does this setting not work?

The symptoms you describe are likely caused by the preference not getting saved to disk. This is caused by a problem with the file permissions, or if the preference file somehow got corrupted.

If this is the case, you can move the preference file (I think the relevant one is ~/Library/Preferences/com.apple.universalaccess.plist) from its location: this will cause the preference file to be recreated.

Of course this will work also for other preferences (though identifying the correct file isn't always as easy); remember to keep the file around (instead of deleting it) so you can restore it in case this fix doesn't work.