Middlemarch.
I've tried reading it three times now, and enjoyed it right up to the point where Dorothea gets engaged to Casaubon. And then, for no reason hit a stumbling block.
It's always the same point in the story as well. I think it's when there's some protracted discussion of the corn laws / agricultural practice.
Has anyone else encountered this? A book you really want to finish but that just defeats you?