Spielberg's not the only person that dialed this in. Harrison Ford's performance was so fucking flat. I mean, it's not like he had that many decent lines to work with, but his delivery just sucked. It's like he wasn't even trying. Cate Blanchett was pretty great, Shia wasn't bad, Karen Allen was okay (although I think seeing her may have just tickled my nostalgia fancy). John Hurt was pretty much wasted, though.
Also, FoC, I just rewatched Temple. I think the movie loses it right after Indy, Willie, and Short Round see the ceremony in the temple, when Indy goes off alone to get the stones. Within 10 minutes, Willie is being sacrificed, Indy is EVIL, and Short Round is a slave. It's like the movie kicks it into high gear, but instead of presenting a narrative, it just gives you a string of scenes that aren't very well connected to each other.