Portal 2 "Skips frames"

Solution 1:

I have done a lot of research on this topic, and this is what I found out:

It is most likely a issue with the game itself. The new update is known to cause problems with Mac users. It should have been fixed by Valve by now, though.

You could also try turning off V-Sync and Multicore Rendering, some people have reported that by doing that, it fixes the issue with Portal and L4D (but some people here have replied to your question with the suggestion for V-Sync).

I have also stumbled at many of those who suggest validating files, etc. (But you say you already did that)

The last resort is re-intalling the game.