Paul Rudd's accent

What is Paul Rudd doing with his voice? He speaks really haltingly and enunciates queerly and it is very distracting. At first I thought he was having his character speak in a refined manner, but after a few seconds it just sounds idiotic. Also, he slips into speech much closer to his usual at at least one point (before his girlfriend breaks up with him.) I didn't watch the second half of the movie; is his manner of speaking explained as some kind of affectation or impediment, or is it not addressed?

Conversely, Reese Witherspoon, despite speaking in her usual voice for most of the movie, occasionally slips into some kind of low-end or Southern accent, such as when they are at the restaurant and she asks if it's an Italian restaurant. This is more understandable, since she is from the South and that was probably her actual accent slipping through. As with the perplexingly low-end shots that litter the movie, this is one that should have been edited out due to its incongruity.