Symptom #1: Footage Flickers or Ripples
"When I scale clips below 100%, the footage seems to flicker or ripple."
Interlace artifacting is not limited to freeze frame effects and still images. It is also a common occurrence with pan-and-scan animation of high-resolution images. When you create graphics with lines as thin as a single interlaced line, this can cause flicker or twitter when the frame is sent out to an interlaced format like standard definition video. Although the image may look completely steady on the computer monitor, it will twitter and flicker on a real video monitor. For that reason alone, it's important to monitor video out to a real video monitor.
This phenomenon is common with animated pan-and-scan stills, because typically the image you start with is scaled down from a higher resolution. The result is that some lines in the image become smaller than you anticipated and reach the single-scan line limit that yields interlace flicker.
If you are getting this flicker, but for design reasons you cannot enlarge the image itself, apply a slight blur to the image using either the Gaussian Blur or Flicker filters. Although you lose some of the detail, you will also lose the distracting flicker. Keyframing the Gaussian Blur to increase the amount of blur as the image shrinks and begins to flicker will give you a more inconspicuous blur.