Fonts on my iMac randomly become blurry (badly anti-aliased) until restart

I have a late-2009 iMac with Yosemite 10.10.2.

Recently, I've noticed that some system fonts randomly become blurry, as though there's a problem with the anti-aliasing. This affects:

• The OS X Menu bar


• The right-click context menu


• The side-bar in the Finder window


• Etc.

In this screenshot - you can see how the text in my browser window looks sharp, but the right-click context menu items are blurry:

They are not blurry when I restart. At some point, usually with in a few hours, and not after any specific, repeatable event - they become blurry. (I've never noticed exactly when it happens. All of a sudden, when I stop focusing on my work, I notice that they're blurry.

If I restart the Mac, they become sharp again, but only temporarily.

I've never played around with any settings that might have caused this change (e.g, I haven't changed LCD font smoothing or anything). And I haven't replaced the graphics card. In short - I haven't changed anything that I'm aware of.

Thanks in advance for any advice or insight!