I've seen 2001 around 5 times, and for 4 of them I've watched it in 2 or 3 pieces. It's understandable (and fortunately the film has well defined 'break' points. I do the same thing with Citizen Kane. Clockwork Orange has slow pacing at times too. The Shining is just amazing. Eyes Wide Shut was a work in progress, sadly.
Our attention spans have been destroyed by television and the internet. Pacing in movies has accelerated so much (cuts, edits, abbreviated plot, etc) that now it's really hard to watch some older films. 70's films are particularly affected because they had glacial pacing. I recently watched Sharkey's Machine (Burt Reynolds) and there was like a 10 minute stake out scene of someone eating, sleeping, etc that nearly put me to sleep. I mean, it did put Reynolds to sleep at the end of the scene. Something like that would never make it to the final cut today.
Oh, and I wasn't kidding about Ebert's commentary on Citizen Kane. It's brilliant. There's a million things happening in that film from a 'newly invented in film' perspective that modern eyes simply can't see because we're so used to it. Having Ebert point out the historic relevance of every scene was eye opening.