why does text display badly on my macbook air

Refurb Macbook air from Apple, connected via apple-supplied displayport-DVI adaptor, to Dell U2412M monitor. Text looks kind of rubbish around the edges, almost as if the monitor was set to the wrong resolution - but it's not, it's set to 1920x1200 which I'm sure is correct.

I tried to take a snapshot of the screen with Cmd-Shift-3 to post here, but oddly, that came out perfectly (i.e. looks way better that the monitor looks in real life).

I also tried this gem that I found on another thread:

"defaults -currentHost write -globalDomain AppleFontSmoothing -int 2"

This seemed to have no effect.

Anyone have any other ideas? I really don't want to be sending the monitor back, but something is clearly wrong somewhere.

Thanks in advance for any help

- Dave