I’m assuming you’re not fighting with this problem after 6 years, but as this thread appears in search results I thought I might be able to help people with similar problems.
What you’re probably experiencing is a text layer that has had its blending mode set to Dissolve. This can happen if you accidentally mash the wrong keyboard shortcut (shit + plus) which will quietly switch your layer to the next blending mode. The Dissolve blend mode has a very distinctive look of rough, pixelated edges that make you question the composition preview settings unless you’ve made and discovered the true problem in the past.
-Matt Henry
@fancymatthenry