Weird Font Smoothing Issue

Question

As the title says, I'm seeing a weird font smoothing issue. It appears to happen randomically, mostly in Safari. I don't know exactly how to explain it, so here are a couple of screenshots from Safari:

Normal screen:

After the problem:

As the images show, it appears that font smoothing is simply disabling itself at random. It happens out of nowhere (the part of the screen where the text is quickly blinks and goes to image 2). Usually if I just reload the page (in this case) it goes back to normal.

I started noticing this a couple days ago. I'm using Snow Leopard 10.6.7.

Any suggestions on what could be causing this?