Personally-I think Romo's downfall in Dec was that with all the positive publicity being thrown his way, it all of a sudden got to his head, and he started doing a tad too much.
Throughout Nov in the Carolina/Wash/Arizona/Indy/Tampa games, he just TOOK what the defenses gave him. He didn't try to do anything fancy or cute, but he did just that.
I guess with all the onslaught of publicity Dallas continues to get(whether they're great or horrible), it may have gotten a bit too much for him. IMHO-if Tom Brady were here in '01, he might have reacted the same way.