|did you change the resolution you were running at?|
like, did your old monitor run at 1280x1024, and now you're running at 1680x1050 or something like that? That could account for your change in performance.
The only other thing i can think is maybe your old monitor was connected through VGA (blue 15 pin connector), and your new monitor is connecting through DVI (white connector with more pins).
are your graphics card drivers up to date?