Here's what you do - at least, it works on my boxes:
1. Bring up the Settings screen (push the Xfinity button on your remote, highlight the gear on the right side, and press "the Select button")
2. Select "Preferences"
3. On the next screen, select "General"
4. Go down to where it says "Prefer Best Available Resolution", and press the Select button to change it to Off.
5. Press the Exit button.
What is happening is, by default, if you select an SD station, Xfinity will switch you to the HD version of the same channel.
At least, that is what it sounds like is happening to you...