You're being a bit short sighted Choo! Costa is a very consistent player, don't take what's been going on earlier this season as proof that he isn't. It's an anomaly really. Costa will still end this season with 15-20 goals and that's impressive considering the season we've had as a club here.
During his time here at CFC, apart from before Guus arrived this season, Costa has been a consistent player that has played at a high level. It's unfair to say he hasn't.