As the only person who can actually answer the question as to whether Beckett is wearing down is Beckett himself, I'll just speculate that I've always read how any pitcher who reaches a career high in IP (or jumps 50 IP in one year), is at risk to break down, slump, etc. not only for the remainder of the year, but also next year.
So, due to his previous blister problems, Beckett is in that group this year - which would be my guess as to his recent struggles. However, since not every pitcher ends up slumping (e.g. by all accounts Smoltz should have slumped), there's no guarantee that Beckett isn't in the minority of pitchers that can handle the increased workload without issues.
So, I'm guessing he won't be as dominant the remainder of the season. But its nice that he'll still be out there because Beckett at 80% or so is still better than some of the options on my team.